Tube amps under $7500

re: "For pairing experimentally with various tube and SS preamps."

Some members may be running right past this requirement in your original post.

If I were in your shoes [IF  this is truly a requirement] I’d probably be looking at manufactures also offering a nice matching (primary) preamplifier that you’ll enjoy along with your selected amplifier(s). Matching up the right preamp matters too, many will chime in here or just look at old threads on this topic.  Next is your balanced connection requirement, which limits the field as others have already stated. With your stated $7500 budget limit set for the amp(s), does this change overall spend equation some if the budget for the [balanced] preamplifier is also brought into discussion now too?

Mixing and Matching:

Like others here, I too pair up my SS and tube preamps with SS and tube amps. Even with good-full Class A 50w SS, in the end solid state sounds like solid state, and tube sounds like tube. Sure, mixing types of each produces a nice hybrid sound. @hickamore gotta ask, can you see yourself also drilling in a little more on your choice of preamplifiers now too?

While looking at amplifiers, maybe take some time and focus to look at good preamplifiers, before selecting your primary tube amplifier. In my systems, the preamplifier choice has a notable impact on the result and enjoyment. One thing is for sure in my system, when I connect my best tube preamplifier into the mix, something really nice occurs about the sound. Matching with the amp matters. My .02 cents fwiw.

Ask:

Do you also have a sense as to what manufacturer’s PREAMPLIFER and model you might choose as your primary preamp for your system?

@hickamore fwiw thought I’d mention that I’m doing what @paulcreed recommended. Currently running a different brand 6SN7 tube preamplifier with a full Class-A 50w Pass Forte’ 3/1A (upgraded) solid state amplifier as my secondary system. Speakers are 92.5db efficiency. Funny thing is I’ve kept this tube pre and ss amp combo in place past six months giving my tube amps a long rest. Or, that’s my excuse, and yet I’ve enjoyed listening to it for while now. Offers a nice hybrid sound and while biased in full Class-A, it manages to hold my attention better than expected. These very long rotations truly allow me to appreciate differences.

@hickamore estimated 1:106 as per spec in its prior and upgraded form following your math ratio approach. While I'm not near the gear or have a means to measure anything, its a guess at best. 

Maybe I should restate this. Whatever is in play for six months (like it is now), is my primary system, LOL.  However, when push comes to shove and I begin parting with my two systems for room and system changes, it's going to be really tough to let go of either of the two sets of amplifiers. Maybe both of them are primary whenever they are in play. :)  Each offers its own enjoyment in different ways.