Preamp for under $400 with tone/bass adjustment?

The loki is just a tone control I think, not a preamp. As I said in the other thread your system needs a better cartridge and maybe some loudspeaker positioning changes rather than new electronics or equalizers. Electronics to cure or change the sound is sort of a last resort after you have tried everything else.

BTW: What loudspeakers are you using? 

ineedhelpwithcables
I have a Rogue Tempest tube amp and a $150 NAD preamp and a Uturn turntable. Does such a preamp exist? Or do I need to have yet another piece of equipment in the mix, like this? https://www.schiit.com/products/loki

The Loki is a great tone control tool, just put it between your Nad and Rouge amp which can be used as a poweramp only
  
Or what I would do, even better ("maybe" with special made leads) put the Loki in-between where the Rouge jumpers come out, then you’ll still be able use it’s source and volume controls (and forget/sell the Nad) and get a Shiit phono stage and plug it into one of the Rouges inputs

Rouge Audio:
This amp has the usual passive-selector-switch/balance/volume-control arrangement ahead of the power amplifier proper. Additionally, it has jumpers that can be changed to access directly the power amplifier input, bypassing the passive front end for external preamp use.

Cheers George

The Parasound Z Pre is a very nice pre in your price range when used, with tone controls.

Yep, as Eric said. I have one and like it a lot. Great value.

Check Safe and Sound, they occasionally have open box, full warranty, for sale under $400. That’s where I bought mine, for right around $360 IIRC.

Just bass and treble adjustments, but nice to have.

Or look for a used Dynaco PAT-4. Bass and Treble for each channel plus loudness and filters.

I thought you sorted the bass on your system with the MM/MC switch...