Make sure the speaker cones arent dry rotted, cracked, or blown and that the voice coils are in good shape. Open the back of the cab and ensure that there is no deterioration of the wood (errr....particle board) and check it externally as well for any major damage. My cabs are pretty beat up, but still sound great because all of the dings are cosmetic. Be careful removing the back plate...the Marshalls Ive encountered have had pretty short wire leads. Another thing to check out are the circuit boards they use on their inputs...Ive heard of a few that have gone bad. I tend to remove mine and wire around them to avoid the problem...plus it seems to give a slightly truer tone (though thats probably my imagination).