It's not the BT label that's the problem.
The box provides what a PaBX wants to have delivered to its power input - a suppressed power supply, which means that the PaBX won't get killed by mains spikes.
Nowadays it's more common to have telephone systems hanging off a UPS.

I have yet to encounter a phone system where sound quality (in a musical sense) was a primary consideration.
Speech clarity is the only that matters.
Telephones are very narrow bandwidth, and digital electronics has improved transmission quality.
With digital PaBX equipment your speech will be processed at something like ADPCM at 4 bits and 8kHz sample rate - remember that CD is PCM at 16 bits and 44.1 kHz sample rate.

I'm a telephone engineer, which is why my head is full of such sad and "interesting" information.

If you want to find out what recorded music sounds like with all the life sucked out of it then the BT power conditioner is your first port of call.
My own experience bears that up.

My journey was I heard via forums that one can suffer with mains borne problems that affected ones sound. Hadn't noticed anything untoward but being of a curious nature I got a Tacima extension supposed to reduced interference. It affected sound OK, it stifled it!

Then encountered the concept of regenerators and had experienced these care of work with IT making servers resilient. They ensure a consistent power regardless of what was coming from the mains. I indulged in a Power Inspired AG1500. That I think ensured the system sounded the same, i.e. not better late at night effect. It taught me how lucky I was with my power supply how lacking in the problems that others try to fight. I think I could get along without it but now it's there it stays.

I have a substation nearby feeding just homes as best I can tell. My neighbours don't have noisy devices, my thermostats and other items, boilers, fridges and the like just never make themselves known. So luckily I have no journey to travel. Yippee

You're welcome

Late to the party as usual..
Unless that is, I'm starting the party

I have experience of the Vertex AQ Jaya.
They make RF EMI vibration drain thingies that plug into the mains or phono sockets etc..

They put their vibration drain stuff all over the place in kit as well.

I got a Jaya and a couple of the phono boxes ones as a thing to get me interested / courtesy when we did some work together on a speaker project.
Each phono box had two inputs.

I got to keep the Jaya. Can't say I really miss the phono boxes.

The material was developed by the two main people in the company from a sonar energy background. It absorbs and dampens is my understanding.

We listened to my kit with and without.
I obviously wasn't listening for the right things.. the vertex guy wanted to add more and more boxes. Thought it was getting there with what was plugged in..

The Jayer I'm not sure about..
I've had it in so long I'd forgotten about it..

Have you recently tried an all out, all in, comparison Martin?

My system has a dedicated line from the mains board. All I could afford at the time was oven cable.

Amp using Mad Scientist Nitro power cable directly into the power socket.
Other equipment will plug into my new purchase; a short Mad Scientist hard wired into an Eichmann power strip, into the other outlet.
iFi AC iPurifier plugged into the strip.

Russ Andrews 'The Silencer' plugged into the computer/modem/wifi router power board.
MCRU Mains Filtration plug used into the fridge socket.

Ferrite chokes on all cables with SMPS power supplies.
Throw in an Akiko Audio Fuse Box tuning chip on the fuse box.

Noticed an improvement with the chokes, 1st Mad Scientist Nitro, iFi AC iPurifier, Akiko Audio Fuse Box chip.
Haven't got a chance to evaluate the Russ Andrews 'The Silencer' and MCRU Mains Filtration plug, but they are in.
Waiting for the 2nd Mad Scientist Nitro to come.

Most of these purchases have been after discussion on TAS.
And have lifted my system to being quite good (to my ears).
Super grateful to you guys for all of your thoughts, advice etc.

Wondered about a grounding cable for the iFi AC iPurifier?
Is there anything else I can add?
Can't afford or find room for a big unit/conditioner. All needs to be small.

My RTZ grounding box plus the 4 cables each to Pre Amp has given a huge improvement.
Cables tried one at a time and a step up each time.

Ah right
How does that work ?
Apologies if I missed it elsewhere

Taking that one step further if you use the spdif then connect your USB inlet of the SEGy to a grounding box, quite scary especially with the lower price equipment where ground planes are not at the top if the designers criteria. Also the SEG is not directly connected to earth but via a psu it is receiving a + 12/15vdc rail the psu is the earth connection

You can also ground the cases out if you wish to grounding box via loosening a case screw//bolt and connect via a small spade. If I were doing the Earth grounding I would use any other pathway imho