Need Amp Recommendation

There are a lot of pertinent things that you haven't touched on such as what source or preamp you're using and how much power you need. I'll have to assume from your amp choices thus far that you need a lot of power.

So without knowing anymore about your system my best recommendation would be to use your Theta monoblocks but look for a good tube preamp to bring out that midrange without softening the bass and highs too much.

I personally like the 6SN7 tube preamps, but it needs to be a very good one if you want to keep it tight. I think even the best tube preamps will soften (and probably plump) the bass slightly.

But this was the best compromise I could find with my NuForce Ref 9V2 SE monoblocks (which sound like magnificent tube monoblocks when used with a great tube preamp).

I've owned VTL gear, & respect their products, but if you're not using tube-friendly speakers, like Merlins perhaps, you're gonna go broke, or crazy, trying to use a tube power amp with speakers that are power hungry.

I could suggest used C-J SS amps, like the MF2500A (which has plenty of power, and is very "musical"), but I don't know those speakers.

How about a hybrid like a BAT?

Are your Citadels originals or 1.5s? If originals, you may want to contact Theta and discuss the 1.5 upgrade, it is supposed to be significant. Just another option.

My Citadels are 1.5s. The preamp I'm using at the moment is an ARC REF 3. I don't find it overly tubey. In fact, it has leaner midbass than my mid-fi home theater processor (Denon's $6k flagship pre/pro).

I was wondering if swapping the preamp for something with more lush midrange might be a solution.

Sources are CDs (fed into Benchmark DAC) or analog LPs (VPI Scoutmaster).