I suggest that you contact Upscale Audio, since they carry many tubes and are very knowledgeable
on your subject.

I had the exact same integrated from Cayin - it's a nice amp. I used NOS Tung Sols in the pre section and they were dead quiet with fantastic tonality. I also ended up with the Gold Lion KT88s and felt they were too loose and warm, with slightly degraded detail. I also tried the modern Tung Sol 6550s, which were a tiny bit better, but then transitioned to the Winged C 6550. That was the best set of power tubes I ever heard in that amp. Neutral and accurate, without that bloated warmth. They are still in it to this day - gave it to my younger brother, who likes it quite a bit.

If you like EL34s, the Russian Electro-Harmonix EL34 which were used as OEM tubes in my VTL IT-85 were very nice, and in terms of NOS, the RFT and Mullard EL34s are said to be superb. The Mullards are getting pretty expensive, as are other NOS European versions, but the RFTs are still reasonable. I'm trying the Gold Lion KT77 in my IT-85 right now, and although it has a nice full sound, I miss the edge and accuracy of the EH EL34s. I plan to put in a set of RFTs, or return to the EH set - will keep you posted.

The amplifier would operate better with a flatter load impedance. The load impedance at the transformer secondary reflects through the transformer to the primary as the square of the turns ratio of the transformer.

Below is the impedance response of the Monitor Audio Silver RS6.

It would be nice if the loudspeaker manufacturers would begin selling optional conjugate filters for their loudspeakers that flatten impedance for use with tube amplifiers.

https://www.stereophile.com/content/monitor-audio-silver-rs6-loudspeaker-measurements

I replaced old kt88 tubes with Sevetlana 6550c and Tung-Sol 6sn7. It sounded much better than the old tubes. But, the biggest improvement and surprise was when I invested the 2 6sl7 with Sylvania vt-229/6sl7gt ($100 ea. Kavin's stash). The amp went from a good amp to an unbelievable amp. I used to own Mcintosh MC225 (regreted to sell it) and Marantz model 9 (1990 reissue). I learned from then that the tubes used would affect the sound of the amp no matter how good the amps were.