This is the place to get the good impulse sets for lexicon, alesis and others.
http://rhythminmind.net/STN/?page_id=4188
As I said I havent tried them all yet and I wouldnt say thay are as good as a rack
unit but with allot of tweaking they can be made to sound good.
For Rack units, There are many of the used ones have allot of ambiant noise and
road wear so they may require a good tuneup to make them usable for recording work.
I'm an old analog guy and love racks units. Even if the sound quality isnt as good
on some and its a pain in the ass to plug them up, I still like using them.
One I found to have excelent verbs, echos and chorus etc is this.
http://www.behringer.com/EN/Products/DSP1024P.aspx
Place it after a guitar or vocal preamp or reamp the tracks with it and it
really does well.
I know its a Behringer and all Behringer stuff is supposed to be junk,
but the difference between a 24 bit unit and a 16 bit like an Alesis is definately noticable.
You dont have that anoying noise floor and ambiant noise to deal with when you're going for really deep reverbs.
Preamps are very clean. Id say if anything, The beringer doesnt have any really unique sounds. They are fairly
generic but the frequency responce and clarity are top notch for the price. It also has some great octavers/harmony
comp and other effects too.
When you mix you can get away with lower bit units. Is not like you're using 100% processed effects on tracks.
Most of your sound is bypasses and you may use only small amounts or purposly use their coloration/degration as an effect.
If I was needing to use 100% effect then I'd want a 24 bit processor. Otherwise its all a matter of taste.
If you do troll EBay for units you can find plenty of used units for under $50 regularly and its definately worth the money.
Behringer also makes an FX2000 and DSP2024 which are newer versions. Those run about $129 new. You can probibly
pick up a good Lexicons used for about the same price if you dont have one.
http://www.samedaymusic.com/product--BEHFX2000
http://www.behringer.com/EN/Products/DSP2024P.aspx