I have a Crate GX-80 in good condition (along with a number of other makes/models of amps). I don't use this amp often and recently loaned it to a fiddle player. He told me that the amp makes a thumping noise when he is playing on the low strings.

So I've done some tests and I've noticed that when I lightly pat the strings of my guitar with the palm of my hand, the amp makes the thumping sound. When I touch my finger to the cone of the speaker (open back cabinet) and lightly pat the strings, it seems the excursion of the speaker is quite excessive. I believe the fiddle player is dropping his bow on the strings while playing and the amp doesn't seem to be filtering out low frequency transients.

I have the schematic and would have no issue changing discreet parts.

I am looking for your input on two questions:
1) Does anyone else have a Crate GX-80 that has the same issue (or is it just mine)?
2) Your thoughts on how to suppress the low frequency transients?