If you're talking rack mounted preamp-i'd say a SansAmp RBI/RPM, or a BBE Bass Max [solid state]. Those to my knowledge go for about $300 new, yet can be had for less used [check the boards here-this is where i've bought my 2 preamps].
If you're talking outboard pedal style-the sansamp unit has received good words. Also-depending on price limits [econo-doesn't really tell us much, a max price would be a benefit].
Onboard pres-i've only had experience w/the stock EB MM Ray5 circuit and the SD MM5 circuit. I like them both, yet i would like to try some other pres in another format.
That's all