The resovoir caps.....I'm the type of person who thinks you got the kit cause you liked it and rather than shell out a load of dosh for some new kit you would rathervstay in with a hot soldering iron.

of course this may not solve your problem with the thump....but the crimsons must be the best part of 20 years old ?  Any cap of that age has started to dry out ...the also have a limited life span. This is dependant upon the spec of the cap what voltage they have been run at and their age.  Most caps are good for a hundred thousand hours or so ...assuming a conservative usage within the middle of there spec.   If you listen say  four hours a day everyday for 20 years ....the caps running above spec you could have them run the equivalent of 100,000hrs easily. 

As to swapping them ...its relatively simple but get the best you can afford.