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.
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.