Has anyone replaced tubes in Aesthetix Rhea ?

Sounds like a tube (or tubes), Stew. Only way to know is to swap 'em and see. Let us know how you make out once you change them.
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Hi Thomas, thanks for sharing your similar experience. It could also be that there was some break in occurring in the tube sockets. I've heard of this happening with a new unit where the connection of the sockets and the tube pins needed some burn in time.
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Hi Stew, glad to hear the hissing sound was not present last night. In my experience, tube noise can be intermittent - from one listening session to another. It can be a little unnerving. I've made a practice of taking a deep breath, powering down the system, and coming back a few hours later or the next day. Sometimes the noise completely disappears, sometimes not. I once lived with a really irritating noise in a driver stage tube of my amplifier for a couple of weeks, then it completely disappeared and never came back. What you're hearing is more likely an intermittent problem with a tube than a problem with the unit itself. Good luck, I'll be interested to hear how your journey progresses.

"...since I can't imagine a tube going bad then miraculously repairing itself."

And, funny thing here... The system is sounding better late last night and again tonight with the Telefunken 12ax7s still in place. Maybe the 15 hours of playing time leading up to a "critical" comparison listen was not enough break in and the additional 10-12 hours is making a difference for the better. We'll keep an open mind and see how things play out.
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Stew, if your Rhea is less than a year old, check with Aesthetix about replacements. They guarantee the tubes for a year. The first gain stage will be the place to first hear some noise as the tubes age. These first gain stage tubes are very critical given the extremely high gain being delivered by this phono stage, and the stock tubes for these positions are hand selected at Aesthetix for low noise. If you decide to try vintage tubes for the first gain stage, be sure to get some help from one of the better dealers who can match for very low noise, like Andy Bouwman at Vintage Tube Services.

I own an Aesthetix Io Signature and have been running it with stock tubes for the past four years. Many people suggest Telefunkens for the 12ax7s in the Io, and I suspect the same would apply to the Rhea given the similarity of the circuits. Others have liked the French Mazda long plate tubes. Both are fairly expensive. Just last week, I finally tried rolling the 12ax7s in the Io's second and third gain stages, just to see what a set of Telefunkens would sound like there. I'd used Telefunkens in a previous phono stage and liked them there. So far with the Io, the jury is still out: mostly a negative change for me in terms of resolution, transient response and tonal balance. I'll additionally try changing out some of the other tubes (a 6922 and a 6sn7) before taking out the Telefunkens, but at this point I don't have nearly the great sound quality that I did have with the stock tubes.

One truism about rolling tubes: you WILL get a different sound. May be improved sound, may not be.

And I still want to experiment with the vintage tubes because I believe some combination of vintage tubes is highly likely to sound better than many of the current manufacture tubes, as described once again by Albert Porter in a recent post on the cost of NOS tubes.
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Stew, I don't have an answer for you. Could be you're getting some sort of power line interference or RF interference. That would account for the intermittance and for hearing it simultaneously in both channels. The interference could be coming from some other appliance running, such as a furnace motor or freezer compressor. One way to check that would be to start turning off circuit breakers the next time you have an occurrence of the ringing.
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I wouldn't assume the power regenerator eliminates the possibility of interference from some other electrical equipment. If I were you, I'd still explore the possibility.

I agree it seems unlikely, but it is possible that some component within the Rhea is failing and causing intermittant problems (e.g., some component within the power regulation circuitry). If replacing the tubes does not resolve this, I'd recommend contacting Jim White at Aesthetix directly to see if he has any ideas.
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Stew, imo you're not going to get it quieter by replacing the stock tubes. This is particularly true of the 12AX7s, and most particularly true of the first gain stage. Jim White has gotten the "low noise" issue pretty well nailed with his selected stock tubes. You WILL change the sonic character of the Rhea by swapping the various tubes, but you won't make it quieter in the process (except to eliminate the ringing if that's an intermittant tube problem).

FWIW, I've spent the past two weeks swapping vintage tubes into my Aesthetix Io and listening to the result. Tonight I pulled all of them and went back to all stock. The sound changed with each change of tube, but not for the better overall in complex orchestral music. I may play with this again later and change my mind, but with our local audio group coming to my house this weekend for a listening session, I wasn't about to leave the system with the sound I'd gotten thus far. And, clearly, others have had different experiences.
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Glad to hear this, Stew! Congratulations on finding a solution, and thanks for sharing the results.