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).
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).