When my last band hung it up, I was using a rack, but only because I have an Ampeg BSP preamp with a clean channel tone that really sounds home-sweet-home for me. Nothing more than bass, mid, and treble controls and I've never even gotten into using a compressor/limiter. While I love the sound of that preamp, I was getting sick of schlepping around the power amp to push my pair of 2x10 cabs when we pulled the plug. If I use it again in the future, I'll most likely look for a nice light power amp like a PLX from QSC.
Expanding my margins became more about having a couple of effects that were really useful. The two things that I could always keep handy were an envelope filter (I adore my Q-Tron) and maybe some mild distortion or fuzz. I don't have a Big Muff, but I'll probably get one whenever I get a new gig. That should cover it for me.
I did a fill-in last year with a rock/blues cover band and I actually used a little Carvin BX-500, which I had just picked up off the TB classifieds. I used it the same day it came off the brown truck. More eq than I can use (compared with my BSP), plenty of power, and all of only six lbs. or so. It ran MUCH better than I expected and gave me a new appreciation for what essentially "works" in a live setting. I'm not going to ditch my rack preamp, but I can also do plenty with nothing more than this little Carvin and maybe an effect box. Either route lets me make decent live sound - that's square one.
What you may find is that while certain gizmos give you a really nice tone or cool effect, those subtle sound differences are often more fun and inspiring in solo practice settings or when you're recording. Live sound may demand a constant helping of a real meat 'n potatoes tone at healthy volume and not much more. The guitars and keyboards get to use lots of colorful stuff, but only because we're driving the bus, right? If you want to noodle with several options without going crazy, I'd say try a Line 6 Bass Pod or maybe a multi effects box from Zoom. Both should do a lot and be rather affordable. If you can put a certain sound to use in a practice session with the band, it's probably worth a try at a gig.