Sure, I like questions like this.
I started out with my first Tascam 244, in 1982, then in 1983, I scored a Tascam 38 & M30 mixer. Then, FF to 1997, and I scored a new 424mkII. Since 12/00, I've started buying vintage peices of recording gear, and have since literally become a Tascam collector. I now have several Portastudios, R/R's, Syncasets and mixers, almost too numerous to mention. I've done maintenance on many of them, and that's how I got most of my knowledge about operation and repair,... from first hand experience. [I do computer repair for a living, but do recording and recorder maintenance for a hobby].
BTW, Portastudio is a Tascam-trademarked name, and all the others, [Fostex, Yamaha, Marantz], are portastudio-like, "me too", clone devices.
The Tascam 144 is the seminal, first generation Portastudio. It is a 4 channel/2 buss mixer [L/R- STEREO], [LINE OUT and AUX OUT are bridged STEREO OUTPUTS]. It has [4] 1/4" input jacks, Trim, 2 band EQ's [fixed, hi/low], a MONO AUX SND, a STEREO AUX RCV, a MONO TAPE CUE [may serve as 2nd AUX SEND], and [4] VU Meters. It runs double speed [3-3/4 ips], and records UP TO 2 tracks simultaneously, but uses DOLBY noise reduction, making it NOT compatible with the subsequent, newer Portastudios. Still, the 144 broke new ground in recorder design, and was the first Portastudio, ever.
The 244 is the 2nd generation Portastudio device. It's much nicer, sleeker, & more refined design than the 144. The 244 has a 4 channel/2 buss mixer [L/R- Stereo], with [4] 1/4" Inputs, TRIM, 2 BAND PARAMETRIC EQ's [hi/low], STEREO AUX SEND, STEREO AUX RCVs, STEREO TAPE CUE, plus SND/RCV RCA patch points on each channel. It has 4 VU Meters, and 4 TAPE OUT jacks. [LINE OUT and AUX OUT are bridged STEREO OUTPUTS]. The 244 runs @ 3-3/4 ips, dbx, and can record up to 4 tracks simultaneously . The 244 is a great unit, and I love it, [now I have 3]. I've done a S***load of production on the 244, and have even used it as a PA mixer for live and live-recording apps. Yes, the 244 is comparable to the 246 in sound quality. A nice, used 244 is ~$150-200, and worth it. You could possibly get one for less, it varies.
The 246 is the 3rd generation Portastudio, and really much more "look and feel" of the 388, rather than the 244. The 246 does use the same 'form factor' as the 244, [similar case, shape, footprint, but slightly larger than the 244]. The 246 is a 6 INPUT/4 BUSS mixer, with [6] 1/4" inputs, TRIM, 2 band Parametric EQ [hi/low], [2] MONO AUX SENDS, STEREO TAPE CUE/[switchable to MONO], [4] PGM BUSS OUTs, [4] PGM BUSS INs, 6 VU Meters [1-4 & L/R], and [4] 1/4" {stereo TRS} effect-insert patch points on each channel. The 246 records at 1-7/8 ips AND 3-3/4 ips, w/dbx and can record UP TO 4 TRACKS simultaneously. The 246 does share many of the same 'features' of the 244, but [except for the case], the 246 definitely has the "look & feel" of the 388. The real obvious upgrade from the 244 to the 246 is the 246's 4 BUSS MIXER [with independent routing and track assign of 6 inputs to 4 BUSSES and/or 4 TAPE TRACKS], as opposed to the 244's "either" 2 BUSS [L-1 & 3, R-2 & 4] "or" 4-direct INs routed to the 4 TAPE TRACKS recording simultaneously. The 2 buss vs. 4 buss thing represents the major difference between the 244 and 246, in design and basic operation. The 246 is definitely more sophisticated unit than the 244.
OK, I do think a 244 would be a fine recorder to have, besides the 246 itself. I've had a 244 for 19 years, and have only recently purchased a 246. Sure, the 244's comparable to the 246. They both use the same 4 track head, etc, and have the same sound quality. A nice, low usage 244 is worth the money, but as with most older units, it may require 'minor maintenance', [i.e., belts & rubber parts replaced]. Mint & near mint 244's can be found out there, but as with anything else, it varies.
Really, I think the answer to the original question [what's a good, comparable unit to the 246] might be... the 388.
The 388 is a self contained Portastudio-like 8 track. It has an 8 INPUT/8 BUSS mixer, with [8] XLR MIC INs, [8] RCA LINE INs, TRIM, 3 BAND PARAMETRIC EQ, [2] MONO AUX SENDS, [2] MONO AUX RCVs, STEREO TAPE CUE, 8 PGM OUTs/8 PGM INs, 1/4" SEND patch points and 1/4" RCV patch points on each channel, [8] TAPE OUTS, with [2] STEREO OUTs [1-RCA & 1-XLR], [1] STEREO BUSS IN, and [10] VU Meters [1-8 & L/R]. The 388 runs 1/4" tape [7" reels] @ 7.5 ips, w/dbx, and recording UP TO 8 TRACKS simultaneously. Definitely the 388 has "look & feel" of the 246, and vice versa.
The 388 would be the next best Tascam device, that would be most comparable to the 246, and also an upgrade to it as well. Going to 7.5 ips and 8 tracks would be a big boost in production value over the 246, but still having almost identical features to the 246, [except more channels/tracks & bigger].
The 388 is a Portastudio-like device, but too heavy to actually merit the name "Portastudio". The 388 is the "Studio 8", and "Porta" is not part of the name. Sure, it's portable, as a self contained, 90 lb. unit. It helps if you have a friend to help lift it. I have [2] 388's, and just bought a 3rd.
In relative terms, you may pick up a 388 almost as cheaply as a 246, if you play your cards right.
Avg price on a used 246 may be ~$200-$250, and avg price on a used 388 is ~$400-$500. The 388 costs more, but is worth more. The 388 can still be found out there, used, some in mint condition, but it varies.
FYI, a brand new 424mkIII is ~$400, and even though it's a great unit, for a 6 channel/2 buss/4 track cassette, you'll definitely get more 'bang for your buck' on a used 388, for just about the same price.
As I said before, some used units may be mint/cherry/rarely-used, but as always, with used equipment, buyer beware.
On Portastudios or R/R recorders, belts & things can be replaced fairly easily, and a certain amount of head wear can be compensated for by proper adjustment. Manuals and parts can be purchased from Tascam. Stuff like this is "fun" for the DIY'er.
The 644, I'm not as familiar with. I don't own one, and can't quote features to you. Suffice to say, Tascam Portastudios such as the 644 & 464, etc, lost my interest, mainly because of the small LCD display. Also, I think the 644 came with an ugrade to a "meter bridge" [LED]. Also, I am not keen on the 424 std, with it's archaic LED design.
I DO like the LED design of the 424mkII/III, it's so much brighter than the LCD displays on other units. I have [2] 424mkII's, and I'd consider getting a 424mkIII. They are well designed units, and I like the 424mkII especially for it's 'flatter' design, and the top-mounted inputs & patch points.
The 414mkII's LED design I find tolerable, because with the 414mkII being a lower-end design, it's still bright & functional, but more of an 'earthier' design. [I don't own a 414mkII].
Overall, I love Tascam Portastudios, recorders, and mixers, and I'd be hard pressed to pick a favorite.
You really pinpointed a unique Portastudio, the 246. The 246 sets itself apart from just about any other Portastudio out there, with possible exception to the 644, 464 and the 388.
I can talk at length, about recording, music, Portastudios, the 388, the entire 30 series, usage and maintenance, you name it.
Anyway, good luck.