I hate to rush to post, as this combo was just installed, but it is so "right" for me from the very first notes I'd like to pass on my find.
The killer combo are some Kuhl tubes I got from Brendan at Tubeworld. Here is the combo-

6H30N-AP=6N30P-DR Russian NOS 1992
Special Quality 6H30, long-life, extra rigid, extra durable DR series tubes have twice the ratings of EB series
$0/pair (more in April)
AND

GZ32=5V4G Mullard branded Philips 1960's
Very good bass and mids, smooth punchy sound, good bloom.
QUAD II amp, Modwright modded Transporter
$160/each
(4 in stock)
0 platinum low noise pairs in stock
I've been playing around with a bunch of tubes for the Transporter, and while they all sounded very good, I had yet to find that elusive "magic" combo I'd been searching for. While I didn't try every possible tube available

Here's a few tubes I've been paying around with:
Stock 6N1Ps and 5U4GB.
Kuhl Tubes from Brendan- Tung Sol 6BQ7 and RCA 6BQ7.
Kuhl Tube Rectifiers- 5AS4 Super Radiotron Australia 1960's and 5AR4 Holland Black Base 1958 Rebranded Mullard Double D getter halos, smooth plates.
Tried various combos of the above, and until today, was really liking the Mullard rectifier and either RCA 6BQ7 or 6N1Ps.
The 6H30-DR and GZ32 combo just opened up my system like no other. The bass and midbass just has so much more texture and detail along with increased weight and dynamics. Dimensionality increased in spades along with a glorious sweet top end.
As I said in the opening, it just sounds "right" to my ears and awesome synergy my other gear and just made the transporter without hesitation THE best digital source I've had in my system. YMMV

Will try to report back as the tubes get more hours on them.
One caveat- The 6H30-DR tubes are lower in gain than the other tubes I've used and where the volume was say "50 out of 103" on my Ref 3, to get the same volume, it is now around "63 out of 103".
Other digital sources I've owned recently- Marantz SA7-S1 and EMM CDSA SE.
Associated gear:
Speakers- SP Tech Continuum 2.5 III
Amps- Theta Citadel 1.5
Preamp- Audio Research Ref 3
Interconnects- Stealth Metacarbon
Speaker Cables- Anti-cables.
Power- Power Plant Premier and Michael Wolff Viola and Bohica powercords. Custom Powercord Co. Model 11+ Series 2 on amps.