Does anyone use this preamp to record Bass? I've been using it for a while, but my opinion is mixed. On vocals it shines. Bass?? Eh.

Any other preamps (in the same $500 and under price range) that you can say from experience makes a bass sound great? I'd probably be selling the RNP, so it would need to perform well with vocals, also.

Or should I just stick with my RNP?

I have a Grace 101 which is around $550 (single channel). I haven't used it for bass, but it's great for vocals (I use it with a Shure SM7B). When I record bass my main signal flow is: bass > demeter compulator > Eden WT300 (preamp) > DI to Focusrite interface. Then I muck it up with software. It sounds good, I haven't felt a need to get another preamp.

I record live bands 24 track simultaneously using various dedicated outboard pre's for all channels. I've currently settled on using my RNP for 2 channels of electric guitar mics (very-nice) and using a couple G/T "The Brick" pre's for both my miced and DI bass channels. I like the Bricks only just a little better for my bass tracks, but their similarly priced and only single channel units so they end up costing twice as much... I've got my (DI-Brick) "Instrument thru jack" connected to a Sansamp RBI and then to the recorder for a third recorded bass track, its very often my favorite sound of my three choices and cost the least...

Your RNP is a dam fine and a very versatile 2-channel pre that I'd suggest keeping around until your ready to spend lots of money replacing. (even then, I'd probably still suggest keeping it)
If you want to try a truly different flavor for your bass tracks, I'd seriously suggest trying out a Sansamp BDDI or better yet the RBI model.

...should I just stick with my RNP?
The RNP is probably the best studio DI/mic preamp deal out there right now. I have yet to hear something that can hang as well with a $2000 Great River and only cost 1/4 of the price.

If you've no use for a mic preamp, ditch it and get the Demeter VTBP-201 that Pacman is selling in the classifieds right now or buy an all-tube head and a Palmer speaker simulator. If you need a mic preamp, the RNP is the one you want until you can afford to spend four digits per channel.

I've used my RNP's DI input for years to record bass and acoustic guitar. It doesn't color the sound, and maybe you're not used to that. I typically don't use the dry signal from the RNP by itself when I do a final mix, usually I re-amp with a mic'd up bass rig and blend the two together or do something similar.

I really like the PreSonus Eureka as a bass pre, mainly because it has usable comp and EQ sections, good gain control. Also, the bypass switches on the comp and EQ make it easy to compare settings, and it can switch the comp-eq order with a single button push. I find mine to be quiet enough, and they make a really nice digital output option for it.

I have been able to try a RNP along with my RNC and it is a killer combination for drums and vocals, and I liked it for bass, but I think that bass players want to hear more presence than these devices give. A recording engineer would probably take the RNP/RNC over the PreSonus.

For bass, my current preference is a JDI into an ART MPA Gold on one channel and a SansAmp RBI on another. The RNP is an excellent tool for many things and worth having around.