You clearly have a ground loop..
RF can ingress for multiple places.. not only your speaker leads...
I suggest you read my paper on "How to Build a Quiet Station"
An audio isolation transformer might solve your problem. You can find a cheap one at RatShack or top quality from Jensen (ISO-MAX). The RatShack has RCA connectors, so you will also need 1/8" to RCA adapters. The Jensen product (professional grade) is available with just about any connector combination. I've purchased a couple of them through Cable Solutions.
Good luck and 73,
I just went thru this exercise. Changed speaker amp from a (tube) WE-300B SE amp
to a Bottlehead Tode (tube) Guitar amp. The Tode sounds great but was prone to
pick up RF. The solution was a high quality audio cable with the Mix-31 ferrites at
each end of the cable. Positioning of the cable had an effect, one had to keep it away
from the power cords. The A/C power cord was ferrited also but that didn't seem to
help. The RFI was reduced but some remained. Fixing a decent ground in the radio
shack really helped (tied into an outdoor copper water pipe) stopped all the RFI into
the Tode. I was also re-configuring the (W3EDP) antenna at the same timeframe. A
new current choke balun was swapped out/in (*). The counterpoise was also changed.
End result, no RFI into the Tode, good audio sound, especially CW / SWL. I was
highly doubtful this could be accomplished as the Tode case has a lot of wood, hence
ingres of RF. But it proved my doubts unfounded.
Follow Howard's advice in his dissertation as best you can and you should be able to
clean things up.
I got a "Ground loop" isolator from Pyle - about $8.75 on Amazon
Free shipping if you have Amazon Prime!
It is a small box with two audio transformers and It has 1/8 inch mini-phone connectors on the wires, so it is plug and play.
No more problem. People who use them with their mp3 players and big car stereos report that it reduces the bass frequencies. But I don't notice it much on the 1500.
Ken - NM9P
This "Ground loop" isolator fixed the transmitter in speaker for the time being. I'll continue to look for the loop as time permits..