I have an ebay search for the word "tube": Category: Guitar > Guitar Amplifiers, Sort: Ending First, Max Price: US $100.00
I used to run it pretty often -- it picks up quite a bit of noise, mostly individual tubes for sale and some new "tube-like" amps, but does a pretty good job of finding things I want to look at -- small tube based practice amps, usually from the 1950's or 60's, including the occasional newly listed Magnatone.
I bought about seven of them a couple of years ago -- ranging from $40 to a couple of hundred (after bidding exceeded my search threshold) - they pretty much have one power tube (EL84, 6V6, or 50C5), and although they all "worked" when I got them, most needed some attention -- I'm lucky enough to live close to a great amp tech, so I got them worked on as I could afford it (usually $50 a session). I also live next to an electronics store that has a lot of NOS tubes. Ultimately, about a $110 average spend on each one (initial price + service + tubes) to get it as good-sounding as I thought it would get.
One of the amps was solid state (could have determined this if I had looked at the picture closely), and one of them just sounds like crap no matter what (it's an AC/DC one from the 40s or 50s -- meaning it would run off either AC or DC power, since either may have been available at the wall -- no input transformer

- beware!), but 5 of them are great, and pretty much sound different from one another. Even the solid state one (Knox - only one knob for volume/on/off) is interesting. So, I have no regrets, except that I'm tripping over amps. Whenever I think about selling one, I can't make up my mind, because the 6 are so different and good, and I wouldn't want to inflict that AC/
DC one on anybody.
If you have a lot of time, a little risk tolerance, don't need a reliable workhorse for gigs, and have access to expert techs and good parts, I think this is a great way to go (I suppose this is a big "if", but you can usually get good parts on ebay, too).
I also have an unmodified Epi Valve Jr. -- It's louder than most of those older amps, but it's too dark sounding for most of my guitars. I have one guitar that's punishingly buzz-saw bright that sounds fantastic through the EVJ. I can also use an EQ pedal to make most of my guitars work with it, but I tend to just use those older amps because they rule
edit: and if you're shopping for hybrids, check out the Vox ADxxVT series (I think it's been updated recently). I've got
an AD15VT that's nice with a Weber speaker and a 12AT7 in it, and I hear great things about the AD30VT. We (here on the board) just sent a reconditioned AD50VT to Falujah, Iraq for ~$300, and I hear they really like it.