You really do need better microphones, but I'm going to skip that to suggest that there is some truth to Beagle's suggestion.
There is nothing terrible about A&H or Mackie preamplifiers, heck even the M-Audio can be serviceable. But they are not in the same league as dedicated microphone preamplifiers that don't cost a lot more.
For starters, let's just skip past the part where every microphone preamplifier design sounds different. They do! You may not be able to hear the differences yet, but you will. AND if you can't then it is a little silly to spend too much on them.
The biggest benefit of a slightly better preamplifier is that it won't make you work so hard. The preamplifiers you own are known to be a little lite in the headroom department. We're not talking about measured headroom - there are too many tricks to measuring headroom vs dynamic range vs S/N ratio, etc. We're talking about a perfect take that is spoiled by a little 'crunchiness' as you exceed the voltage rails of the power supply... not the end of the world, and certainly avoidable, but heck, if you can make the problem go away... (and it is just too darned easy to do this, especially on the Mackie and A&H mixers.)
There are LOTs of stand-alone microphone preamplifiers with oodles (that's a technical term) of headroom, and that's what I usually recommend to folks who ask. Oodles of headroom means you can be a little more dynamic, a little more expressive even, without worrying about it.
Amongst the no-longer-manufactured class I like the Gaines Audio MP2, the SPL Micman (which also has oodles of gain if you happen to be messing with ribbon microphones), and the Groove Tubes MP family. I think you can get any of these for around $150 to $200 per channel, maybe less. That's not a lot to pay for two good - easy to use - channels of microphone preamplifiers. (NB- the Groove Tubes preamplifiers require a Groove Tubes power supply and cable, and some unscrupulous sellers don't include them. If you can find the whole kit they are really cool!)
If you are a DIY type you can scrounge around for old channel strips (I have a pair from a Trident Tri-Mix that sound awesome when they work, but they are not the most reliable!) or old gain cards - I have a bunch of Melcor cards that I've used to build preamplifiers, they sound really nice, almost like an API.
And there are worse crimes than making recording easier.
The problem with gear is that sometimes we forget why cool gear is cool. It isn't cool JUST because some famous dude used it - but it is cool if it brings something to the dance. And I'm as guilty as the next guy, so learn from my mistakes!
If I had it to do all over again I'd start with microphones, and I'd do nothing but gather some really great microphones. What constitutes a really great microphone differs from person to person, but that's where I'd start. Then I'd focus on preamplifiers, and then my monitoring system (room, loudspeakers, amplifier), and then everything else. You can make an argument to start with the monitoring system, but in hindsight I think start at the other end of the chain.
Acoustic guitar and voice can be darned difficult to capture well... but it is so much fun trying to do so!!!!