@ulul 4 Ohms isn't a good load for the SIT amps and is marginal for the XA25. These amps are not high power and really are much better suited for slightly higher impedances since they tend to run warm- lower impedance loads (like 4 Ohms) will make them run warmer.
There are class D amps now that have every bit the quality of sound you seem to be looking for, but no heat (and unsurprisingly) more power. You might want to consider widening your search.