EE Minimax Phonostage

I use an Eastern Electric Miniman Phonostage. I replaced the stock tubes a while back...using an Amperex 12AX7 in the middle position flanked by United Electronics CV4017/5751s..which are basically re labelled Ting Sols from the 60's (fabulous tubes). Currently swapped the 5751's for a pair of vintage GE 5751s (black plates/triple mica)...initially they sounded terrible, screetchy as hell...but now they sound great. I'm not sure what's happening. I don't think they sound as sweet as the Tung Sols, which have a rounder, sweeter tone, but these GEs are very nice sounding regardless.  What are some vintage tubes I should try, both in the middle position and the sides? I understand the middle tube is the main gain tube, not sure what the side tubes do.