I got a engl screamer 50 combo amp,and everything is fine except I've noticed that i'm losing bass,the tubes are around 1 year old so i guess its not them.
If it really is tubes ,is it going to be the big ones or small ones ?
It'll likely be the bigger ones (power tubes). Just because tubes are relatively new doesn't mean that they're not the culprit. Gradual changes in amp dynamics like that are usually tube issues. Mother other ailments are normally immediate. I'm not an amp tech, though.