I have a Boss RC-3 and a Marshall head with two 4x12 cabs. I quite frankly can't stand cramming recordings, bass(not too loud), piano, guitars etc., into mono and I am wondering what I'd need to acquire in order to set up the two cabs as stereo.

Here's what I am thinking:
Head + Instruments -> Mixer -> RC-3 -> ??? -> Cabs.

I don't have the bling for PA's and I don't want to overpower the cabs.

Thanks for your help.
You really don't want to do this. Guitar cabinet speakers aren't meant to project the full (clean) spectrum of a decent monitor-mix. Have you ever tried plugging in a mike into a guitar amplifier? Exactly.

I would save up for a used amp, and some PA speakers. It's really not that expensive.
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Take a look at the Carvin LS-1503 speaker cab:

Possibly one of the great bargains in speaker cabs, this is a cabinet capable of handling up to 800W (and you can put it up on a speaker stand) and runs about $269 (here in the US) each. Full range frequency response and smaller, lighter and louder than the Marshalls with cleaner bottom end and far better sound dispersion, particularly of highs.

You can run vocals, keyboard, bass and even modeled guitar (or guitar from the preamp of any amplifier) into one more more of these all day long. You'll need a stereo power amp (and a mixer that will maintain the separation, of course), and something like the Carvin HD1500 would be a good workhorse at around $299. http://www.carvinguitars.com/products/HD1500

I'm using a similar system (more expensive speaker cabinet, but with the same basic configuration) to run modeled guitar, bass and a Korg Kronos keyboard. While the power may *seem* a bit much, you'll find that both bass and the keyboard need the headroom, and that running a power amp in stereo into a pair of 8 ohm cabinets results in power outputs well below what the amp is capable of in mono mode.

In a gigging situation, you'll mostly find that only a few FX really translate well to stereo in a large room, but for home/practice or small room use, it can be fun.
Last edited by dspellman at Sep 6, 2013,