Need help tuning my wreck

[declan]

Removed cheap test tubes and now have a set of winged C El34 and Brimar preamp tubes which have definately improved the tonal quality. I also managed to pick up an old beaten marshall slant 4X12 last night loaded with 3X Rola Greenback 55hz cones and one Goodmans audiom.

at 1 oclock on the volume I am getting a lot of squealing and feedback when not playing, I know some of this is normal but it seems a little excessive.

Also the feedback seems to take over and shorten the sustain on all but a few notes on the neck. 12th fret harmonics are also pretty dead, again they seem to be overpowered by the feedback. In short I dont think the notes are blooming like they should. Chord work and on clean settings with the guitar vol rolled back are sounding exactly like i think they should

Any advice for the best approach, or order of things to look at to improve her.

thanks in advance

declan