It sounds as though you only have one powered out put instead of a dual powered amp. Since you haven't given me a lot to work with my suggestions are as follows:

1. Get a dual powered amp, in the Behringer PMX2000, Yamaha EMX88S or the Samson XM910 range. All have eight channels, dual power, buses, and EQs to run a separate FOH and a monitor mix, deliver two channels of power in the 300 watts into 4 oms range (which is plenty for your use) and they are all in the $500 range, which is fairly inexpensive for what they offer. Mackies are way too expensive and Crates powered mixers are a bit too complicated for what you discribe.

2. Monitors do not need to be expensive and are often the most wasted money new bands spend. They do not exist to allow you to hear yourself in all your glory, but only to provide cue, volumne and timing to your music. A couple of Peavy 112M monitors (around $180) will do nicely and if turned upside down on the 45 degreee angle will reduce the feedback from your acoustic guitars. They also have seperate volumne controls to reduce the sound to one or the other of the monitors (helpful if you have one or two folks who are hard of hearing).

#3. Get a separate small amp for your bass and then throw out every piece of advice your have ever received about a "back line". Any bass amp behind you will (due to the rounded characteristic of lower pitched sounds) feedback into your vocal mics, making your vocals muddy, and your classical guitar humpy. Even the use of 80 to 100 hz high pass filters won't solve the problem. Set the small bass amp (no more than 10 or 12 inch speakers) at the corner of your FOH speakers aimed mostly at the audience but with a little back towards the bass player. If he's playing a doghouse, he doesn't even need that, but in no case "behind the line". It will project enough bass into the crowd, without overcoming your monitors,, the FOH and your vocals. The Fender Bassman 25 at $270, works nicely with a "line out" back to the board if your need to project more power through the front of the house. If the bass sounds boomy, get the amp off the floor, and onto a folding chair, if necessary.

4. With all of the advice you may get, focus on creating a sound system which incorperates much of what you already have, is flexible to allow other components to mate up (your band will not last in its present configeration forever), can be resold at not a great loss, or bought up by one band member, and doesn't add much to your present gear assembly.

5. At some point you might want to get smaller FOH speakers, but wait until your have a good sound coming out of your present speakers (which you say aready sound good) and then using your final set up try different and smaller speakers into the mix until you hear what you want, at the size you can carry. Actually, you may be able to trade your Crate 15" speakers (my guess as to what you have) into Crate 12"s, which is what I did years ago with excellent results.

Good Luck!