I know there was a commercial design for this some years ago by DPA audio amplifiers for their transistor amps, but I thought one day why not try it out on my then Leak stereo 20 valve amplifier as the feedback connection can be very easily taken from the output transformer to extend it out to the speaker terminal thereby including any speaker lead losses within the feedback loop(it is also very easy to reconfigure back to the original if you don't like it). Although I haven't got the leak anymore I thought the results were very good and interesting,although I didn't try any different wiring in the extended feedback wire I suspect this could have an effect and may be worth experimenting with. There is one important pervisso however you must make doubly sure that the extended feedback cable is always connected with a good connection at the amp and speaker ends otherwise the gain of the amplifier will be increased substantially and may result in damage to your equipment.
With modern amps based largely on complicated circuit boards this mod is not easily carried out, but on older hard wired valve amps this can easily be done .