Tubes in AR PH3 phono preamp

Hi there,
I am new into tubes or even precisely saying I've always trusted and trust to solid state design but it happens that I've borrowed to listen hybrid phono stage with remarkable dynamics and yet softness and soundstage.
I have a small problem while I'm listening: If I listen a music during 2 vinyls playback time, it seems that highs become to dim and on the other side, if I just start listening the highs are bright. Currently I've tested that phonostage on no vinyl operation and it's ultra quiet even on maximum volume setting of my preamp (Bryston 12b)Does that mean that I need to change tubes? Currently I have SovTek 660922 tubes and as far as I know they've never been replaced since 3 years ago. No hourwise info I have currently.
My current analogue setup is J.A.Michell GyroDec with RB300 and Benz Glider (~500hrs), Audio Research PH3 phono preamp, Alpha Core Goertz Sapphire b-n phono and preamp, Bryston 3b-st, Kimber PBJ silver balanced interconnects b-n preamp and amplifier, JPS Labs Superconductor b-n Bryston 3B-ST and Totem Forest loudspeakers
Are you warming up the tubes. Tubes tend to need about an hour on standby to warm up. This could be the difference you are hearing in the initial listening. After that they should stablize. If they sound a bit dim, it could be the tubes, or you may consider changing the loading on the cartridge. By increasing the load you will bring out the high frequencies. I have found that nominal loads are just that--a starting point and nothing more--to really maximize the performance you have to experiment a bit and try different loads until you find the one that's right, and it will change with tube changes, to balance everything appropriately.

As to replacement, I have gone more than 3 years with the same tubes in an SF-phone 1 They are also 6922 tubes. Sovteks aren't bad, but you can really improve things by going to NOS tubes. I recently bought some Siemens (6922)and Telfunken (12AT7) NOS tubes and while they were expensive, they were one of the most effective upgrades I have made. Well worth the $ if you listen to a lot of vinyl.
i agree with abby on the tube (and other electronics) warmup thing. call me nutz but i leave my sp3c on all the time, as i do the adcom 555II and everything else. NEVER has it seemed dull in the highs either in the phono or line stage.
tube rolling is an option and can be quite rewarding. i got a pile of tubes at the trw swap meet for cheeeep so its going to be a while for me. has tubes as does arc is probably where ill go when o, ready.