I got my ICs from eBay seller HA5IA and the amp sounds great to me.
https://www.ebay.co.uk/usr/ha5ia?ul_ref=https%253A%252F%252Frover.ebay.com%252Frover%252F0%252Fe11401.m1842.l1181%252F7%253Feuid%253De16cf49c9615433db2b37278b60bb955%2526bu%253D43117872547%2526loc%253Dhttps%25253A%25252F%25252Fwww.ebay.co.uk%25252Fulk%25252Fusr%25252Fha5ia%2526exe%253D19027%2526ext%253D49342%2526es%253D3%2526nqc%253DACQAAAAAAAAAAgEACAAAAQAAQAAAAAAAAAAAAQIBAAAAAAEAAAAAACIAAAgCJEAAAQAAIAAAIAAAAAAEBAAAAAgAgECAAABAAAIAAAAAAAAAIYAAIAAAEAICUAICkAAIBAEEIAACBAAQBAABAAwQAAACAABAYAAKBAAIACQAAAAAAAEAIIAAAAAAAAAAAAAAAAAAAAAAAAAIAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAEA**%2526nqt%253DACQAAAAAAAAAAgEACAAAAQAAQAAAAAAAAAAAAQIBAAAAAAEAAAAAACIAAAgCJEAAAQAAIAAAIAAAAAAEBAAAAAgAgECAAABAAAIAAAAAAAAAIYBAIAAAEAICUAICkAAIBAEEIAACBAAQBAABAAwQAAACAABAYAAKBAAIACQAAAAAAAEAIIAAAAAAAAAAAAAAAAAAAAAAAAAIAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAEA**%2526ec%253D1%2526sojTags%253Des%253Des%252Cnqc%253Dnqc%252Cnqt%253Dnqt%252Cec%253Dec%252Cexe%253Dexe%252Cext%253Dext%252Cbu%253Dbu%2526srcrot%253De11401.m1842.l1181%2526rvr_id%253D0%2526rvr_ts%253Dd1b1f01e1650a8602960bd91fff7d1e5&ul_noapp=true

Very sensible idea.  If one of my ICs goes again, I'll definitely put in IC holders because I don't know how many times the circuit will survive being blow-torched.

I have had this problem twice.  The amp is pretty old (circa 1986), I don't know what the underlying cause was, but each time I fixed it by replacing either IC1 or IC2.  The part number can be found in tiny lettering on the IC and people sell them on ebay for just a couple of pounds. (It was a bit gutting to discover that this classic cranked Marshall sound is all coming from a dirt cheap Texas Insturments IC (insert emoji of choice here)).

The tricky bit is desolding the IC because it has 6 or 8 legs which all need removing.  My method is to pull on the top of the IC with a pair of pliers, while giving the all the legs a quick blast with a cook's blow torch.  It's obviously risky and I have slightly scorched the circuit board, but I was quick enough not to do too much damage and it worked.

Good luck.
Matt

I have had a similar problem with my Marshall Master Reverb 30 5203 and thanks to this thread I fixed it quickly.  All the posts above are much appreciated. 

On my amp, as I turned up the preamp volume, it was fine up to about 3, then started to fiz and go quiet at 4.  The problem was one of the little IC op amps, in my case IC1, an MC1458, easily and cheaply obtainable from ebay or Farnell.  It was only a few minutes work to replace it, although desoldering the old one was a little tricky.

For future ref, I noticed that while the amp was faulty, turning up the pre-amp lit only one of the LEDs; after fixing it, they both light up.

All the best and thanks again to the original poster and answerers.
Matt