Seeking a Tube preamplifier

Like you, I had a Rogue Audio RP-1, but I then moved to an RP-7 for a marked improvement in sound quality.   Your budget, however, would allow for an RP-9 from Rogue Audio.  Having red the very few r Jews of the RP-9, I am considering having Rogue upgrade my RP-7 to the RP-9, although I would be quite happy to retain the RP-7, as the sound quality is superb.  I am using it to drive two monobloc solid state Odyssey Kismet amps.

I was a rogue guy. I had an old sixty-six from the late 90s. That was a great sounding amp. When I started on my big upgrades a few years back I went with an RP-1. It sounded so much better and I thought to myself, an RP-7 must be even better so I got one. It was crystal clear but somewhat sterile. No tubey goodness at all. I rolled tubes extensively and it got better but never where I really wanted. I read a lot about the don Sachs line stages so I got my hands on one of those and I felt as if nirvana had been reached, but like you, I needed a balance control and the Sachs didn't really have that. What it did have, and what I read a fair bit about after enjoying it, was 6sn7 tubes. I went on a search for preamps with that tube type. There are not ultimately that many: Cary, atma-sphere, modwright, vac, maybe a couple others. It was easy to get my hands on a modwright ls100 and it does have a balance knob that you can turn on and off. This has been where I've landed. It sounded great out of the box and with some NOS RCA tubes it it sounds even better. The Music Room carries them so you can buy it and try it and if you don't like it, return it.

If I were to upgrade again I'd strongly consider an atma-sphere as I ended up getting some used MA-1 amps and they made maybe the largest difference in sound other than my analog rig or speakers.

Consider BAT VK-80.  Balanced.  Line stage only.   It might be a little outside the budget you mentioned but it has the characteristics you described.  I use mine in front of a D’Agostino S350. Amazing soundstage and it really helps a powerful SS amp tame typical SS issues without disrupting the dynamics and detail.    

+1 LTA MicroZOTL preamp.  I’m very happy with mine with the upgraded 6SN7 tubes.

Have you ever considered getting a Hybrid preamp?  I have an ARC,  SP-9 MKll that has a SS input section and a tube output. (Old). I also have a PS Audio BHK preamp that has a tube input section and a SS output section.

Just my two cents.