XLR to RCA Adapters

If going from balanced to single ended you may gain something using a Jensen transformer such as their pro to consumer model but I can't say for sure. Most people just use the adapters from BAT or the like.

I will answer this by saying the folks at Jensen are available and IME very helpful in answering questions about the use of their transformers. They are very good at determining which of their products best suits your goals and then explaining the anticipated result of using the product. I would call them if you are serious.

Regarding channel balance tracking and sound quality improving as you advance a volume control, these may be two different things. The channel balance improvement should only be relative for volume pots (e.g., potentiometers) and should not be affected if the volume control is resistor based or a chip control. However, I suspect there could be instances where sound quality improves as the preamp is operated at higher gain (or less attenuation) because the associated componentry is operating in a more linear range resulting in a lowered noise floor and improved signal to noise ratio. This is one benefit the Rothwell people speak to when promoting their 10dB attenuators. I suggest the EE's or designers chime in on this issue since I am in the deep end now.

Yes, the pro to consumer Isomax gives you the step-down ratio (so as not to have higher output pro gear overloading a commercial consumer unit with no volume control at the input). I'm assuming they'll make this up for you with any combination of connectors you want -- maybe with your own choice of ratio as well... It wouldn't hurt to inquire. If this route sounds good to you, do compare the data pages. Folks with rolled off (up to about -3dB at 20K) NOS chip DACs, or RIAA EQ's down about one dB at 20K might well prefer not to stack another transformer in the signal path whose FR starts to roll off before 20K. I saw step-down options on site rated to 60k, and even 180K, depending on the circuits that bookend the transformer.

One more thought: If you can get custom as easily as the advertised stock items, you might give some thought to eliminating two XLR connectors by having a flying balanced lead hardwired into the Isomax unit at its output. Perhaps Jensen could accommodate you with your lead of choice if you provide 'em with the desired bits; or tackle that end of it DIY.

Another thought, though I am not the transformer answer guy, I do note that these Jensens (and most?) input transformers of this type want to be loaded by 10K up to 25K for these applications. Your preamp may not like to see this kind of load reflected at a 1:1 ratio -- and don't just go by what someone says "will work": distortions, conventionally measurable or not, tend to increase into lower impedance loads, even with [tube] preamps presenting an output impedance of only several hundred+ ohms. So check this out carefully.

Then, too, I'd guess the BAT's input impedance would need to be modified one way or another -- and IT may not like this change. An advantage of the 4:1 impedance step-down ratio shown for the pro to consumer Isomax is that it takes a 10K impedance across its secondary, and reflects 40K back to the preamp output. All that costs you is some overall available gain in your amplification chain; you probably have a surplus of that on tap as things currently stand.
No, the transformer approach is less than elementary when tube impedances are involved.

Oh, and one more thing... You COULD get a cathodyne (split load) type phase inverter pair made up as an adjunct to the inputs on the BAT. This would get you past the issues I outlined above, at the expense of more circuitry (never a free lunch...). You'd need a power supply for that circuit (not too complicated, in this case), and it would also have the downside of needing a very small value cap at its input so that the circuit can be biased. This sort of thing is also known in a somewhat different context as a bridging adapter. Something in the 6DJ8/6922/7308 range would do. Some feel even lower impedance tubes are better for this (5687, ECC99, D3a triode connected, et.al., down to the circa 800-1200 ohm Rp jobs -- there are various choices for the latter, but their characteristics tend to be all over the place, necessitating tube selection.

Astronaut Norm Thagard designed solid state versions of this in order to bridge FOUR (or was it EIGHT?!) H-K Citation II's for his monster tube system. He published a detailed write-up in 'AudioXpress' (then 'Audio Electronics', I believe), the DIY magazine. It all worked out great, reported Norm.

So, it can be done... Are you sure you want that BAT?