I have the 1987 clone and I wonder what other tubes that fits in the socket works with this amp? other than EL34 of course...

You can also experiment with using the 6550 or a KT88 and then use whatever type you seem to like best.  I have two Marshall 1987 heads and over the years in my band we have used: The JJ E34L, Saratov KT66, Valve Art KT66, Electro Harmonix KT88 and 6CA7.  I've even experimented with a set of Electro Harmonix 6V6's just for fun. 

The amps get used through a Marshall Power Brake and predominately rhythm guitar and so far it seems as though the best results have come from either the Saratov KT66 or the EH KT88.  KT88 has a lot of punch and is excellent on stage when using a humbucker.  It's not quite as good with a single coil pickup just because it doesn't overdrive quite as easily. 

Right now we're testing the Russian Genelex KT66 reissue which so far seems quite cool and similar to the Saratov.

These circuits are quite versatile and it's worth experimenting to find what works best for you even if it's not historically accurate.

I guess I should have specified in my post that with each tube type I was always adjusting the bias to get the correct idle current for each tube type.

What's interesting is how "Marshall-y" the amp sounds no matter what output tube is in the socket in the one's I've experimented with over the years.  Including the 6V6's.  Overall tone is not incredibly changed, it still sounds like a Marshall but feel and overall playing dynamics are greatly altered for better or worse and that's why it's worth experimenting.  The 6V6 tube might just be the hot ticket for someone who's amp is way too dynamic for their playing situation.

I have experimented with placing the wire for the feedback loop on different secondary taps of the output transformer with different types as well. 

The little guy 6V6's do look a little weird in the amp but they don't need any tube retainers to keep them in their sockets. 



In my test the 6V6 just didn't have quite enough headroom for the application yet the tone was quite nice and I'd recommend giving it a try in an amp that just has too much headroom.  I've used this same 6V6 in several amps with very good results.  My Marshall was at right around 400 volts which is over the max rated voltage for a 6V6 yet just about the same as the voltage that Fender used in amps like the Deluxe Reverb, good quality 6V6's are a must for this swap.