Stupid question perhaps, but does the Gauss cabinet have a tweeter, and could it be broken? If there's a cross-over inside the cab, the highs will be lost. It could also be that the amp section is really voiced for the speaker that it came with in the combo.
When using any SWR amp, you should be aware that the Aural Enhancer is not a boost/cut control like the other knobs, but more like a master volume (boost only). It is not really an effect, but a filter that does the infamous 'smiley curve' - scooped mids, boosted highs and lows - like the preshape switch on an old Trace Elliott, IIRC. The good thing is that here you can adjust the amount of 'grin'.
Just remember that 12 o'clock on the Aural Exciter is everything but neutral - and to help you remember that, there's no center detent on it!
I turn it up to about 9 o'clock, never higher. If you turn up the volume with a higher setting for the enhancer then you'll lose a lot of definition for the lows, especially if you're already boosting the lows on the amp and/or your Epic.
The situation on a WM15 is a bit more complicated because of the semi-parametric midrange control. Try cutting about 5 dBs at 1 kHz, that usually creates a sound that's nicer on the ears.
I played a Spoiler through a WM12 the other day, and with only the neck pickup on indeed I got a very honky sound. With my Epic, through a WM12, there's no honkiness at all. In fact, I boost the mids just a tiny bit to hear myself better in a band.
Don't give up too soon!