rolling output tubes

I have a pair of Melody M880 monoblocks in my 2nd system. They use KT88s and have a bias adjustment pot for each KT88. Can I use EL34s or other output tubes instead?

Do output tubes and transformers have to be "matched"? A dealer once told me that transformers designed for KT88s will not achieve EL34's potential. But I thought he tried to sell me another amp. LOL...

This is for my 2nd system and is in a smaller room 12X11. The speakers are 90dB. I want something a bit euphonic from this system but not too much. What other output tubes should I try?

The suggested bias for KT88s is 50mA. But I found it sounded the best between 40-45mA, depending on the tubes used. So if I move to other tubes, what bias level should I start with?

Thanks a lot.