The room treatment advice is a no brainer. If the room is wonky you're gonna be grabassing around in the dark.
If you're building your own enclosures I'd try to build something that sounds kind of like a Genelec or Adam or B&W nearfield. A reliable monitoring system that is known to be able to translate detail is important. Checking your mixes on bigger systems with more bottom is important too, but the bulk of the mix process happens on nearfields generally.
Beyond the mechanical transduction barrier, the aesthetic value of "good bass" is going to dramatically affect your mixes. There's no set formula to get you there, but a lot of considerations. Compression is an important one. Especially if you're trying to mix rock or pop music, or something with a lot of statistical density in terms of different instrumentation types, it may be necessary to clamp the lows. Where and how you do this depends largely on what you're dealing with in the first place. 2 buss compression is going to glue a lot of the elements together on its own. Whether or not you need to clamp before that depends on a lot of things, but it can really help sometimes. The track should tell you if it needs this or not.
For example, if you're doing a light jazz recording with someone playing double bass, compression might not be much of an issue at all. If you're trying to get a P-Bass to sound like a Stingray or vice versa, good luck. Not likely to happen. Compression can serve as damage control with bass to an extent, but you might sacrifice detail to get the low end to sit down. Having both is going to depend more on tracking - the player, the instrument, how well the sound was dialed in initially and the capture. Trying to hype anything to do with the bass sound will probably be difficult to work with - eg. really cranking up the low end or something. Bass is powerful. If you have enough it carries. If you have too much it ruins everything. Somewhere in the low mids, like around 250 to 500 Hz is an important range for power. Not enough will sound dead and too much will sound like mud. Having a shade of presence in the upper midrange/higher frequencies is going to affect clarity. If you got a player that clacks the strings or something, you might not be able to push this too much. Or you might have to chop something out to get a useable track.
Fletcher-Munson is important too. Once the room and monitor thing is set up the way you're going to go with it you should audition some reference materials with your favorite bass sounds at different levels. Try to gain a sense of the energy happening in the low end at around 60 dB SPL. Then crank it up to around 85 and see how it changes. Compare it to your own stuff at equal volume levels.
Oh, and lately I've been really impressed with Thomastik Infeld strings!
Also consider if you have low end energy coming from multiple different instruments that's walking on all the other stuff and turning it into mud. Sometimes you can hi pass stuff that really doesn't define the low end anyway - eg. guitars.