2 y/o Blues Jr. (mfg. 2018) major volume loss after about an hour of play. Now only around 10% sound output. Suspecting the preamp tubes, as all 12AX7A’s are shining only very dim compared to another BJr in side-by-side comparison. Wondering about replacement with (3) 12AX7 tubes (sans the “A” - and somewhat less expensive)?