Well I don't have an RNP, but I did just pick up an MP20, so I can give a review of it:
First off I don't regard them as directly comparable because it's easier to get used MP20s and I don't buy anything new. Used RNPs are hardly discounted at all, so the used MP20 is about $125 cheaper.
There are no worries about wallwarts or rackmounts. Ultimately those aren't sound considerations, but such practical matters are important to me. It would be tough for me to accomodate wallwarts in my rack setup. It also has many more LEDs on its meter than an RNP which is nice but not essential.
Anyway, the unit has a couple of other clever features: a stereo bus & a headphone amp. I wouldn't want that on every preamp I have, or even most of them, but it's nice to have on one unit since I only have a virtual patchbay. I can try out mics, guitars, stereo mic patterns, etc. without firing up the entire rack.
It also has one totally useless feature: the IDSS control. Supposedly this give you a tube-like sound. Full disclosure: I am a fan of cheap tube pres for guitar DI. The IDSS does not sound like a cheap tube pre, it sounds like a Poo control. Instead of adding musical harmonic distortion, it subtracts high end in a very displeasing fashion. Thank you, but the tone control on my guitar works just fine.
Basically the unit generates a nice clean sound. Great as a bass DI, tight low end. Not so good as a guitar DI (I prefer my tewbs!), at least with my LP. Probably OK for a Tele.
Unfortunately I have my favored vocal mic, the SM7, out on loan, so I reserve comment on vocals. I tried a KSM33, SM57, and MD421, but I sound bad on all those mics. I accurately heard myself as a combo of Neil Young and Axl Rose. Did it make the SM57 sound like a great mic? A little I guess. I'm not sure I can be objective about that.
So instead I plugged in an SM81 and moved on to steel string guitar. That was very nice, very accurate. I hit +28dB on the meter several times without clipping. Next I tried digeridoo, also successful. Now the SM81 is undoubtedly a great mic and to me just always sounds good unless somebody starts singing. The MP20 didn't muck it up at all.
Basically I would describe this pre as having little to no character (unless you turn up the Poo control). The character of the mics was very obvious. It has a number of handy features. If you are looking for a warm preamp or a sparkly one, this isn't it. I'd describe it as more of a workhorse, which is the role it will play in my studio.
Note: the unit I bought is 1999, and has Jensen transformers. I've heard newer units use something else but I don't know if that's true or if it makes a difference.
