Any replacement tubes for Sonic Frontier Line 3

Try the Sovtek/Reflector NOS 6H23NEB. They are really better and more tubey than the standard Sovtek 6922. You can buy them at very affordable price (45u$s matched pair) at Partsconnexion, Upscale Audio or The Tube Store.
You can contact to Chris Johnson (ex Sonic Frontiers President) at www.partsconnexion.com and tell him which type of sound you are loocking for. They have a big stock of 6922 (like Valvo, Mullard, Siemmens, Tesla.....) and they have many experience in Sonic Frontiers products.
Good luck.

I have changed all the tubes with Siemen CCa E88cc matched pair gold pins in the first pair and Siemen ECF82 for 6GH8A and then 4 pairs of Siemen E88cc (gold pins) from partsconnexion.com that Mr. Chris advise me. After I have changed all the tubes the sound warm, upper detail in high end, more soft bass that I never heard before. You should find the very good tube for the first pair (input tubes) Telefunken E88cc CCa is the best and very rare.

Hi BW. Merry Christmas! Just a note here. Rca never made a 6922, they sourced them from other many manufacturers and relabelled them.And many relabelled Rca 6922's are in fact seimen ecc88. I have seen amperex bugle boys and valvo's relabelled by RCA as well.

Replace the Sovtek 6922 with Siemens 6DJ8/ECC88