astor,
There are many factors playing a part here so i will try to address a couple of them. Others will no doubt jump in.
First off, bi-amping, although an interesting idea, should really not produce a difference in sound, not unless you are using two different amps for each driver section, say a tube amp for one half and a solid state for the other. Some people who have REALLY turned up the volume may get some benefit from having a speaker bi-amped as it may provide more power per driver section thereby giving the speakers more headroom from the amps' capabilities.
Anyway, going beyond that, the speaker impedance is actually best if it is an easier unit to drive. For example, an 8 ohm speaker provides an easier load than the 4 ohm speaker. This equates to less stress on a typical receiver such as your HK 525 which usually equates to a consumer being able to buy much less expensive components for powering their gear. Those who seek out 4 ohm (or less) speakers tend to need more expensive amp sections if not separate components altogether. Nonetheless, since you have 2 units available for powering this may not be an issue. However typically when one biamps a speaker, they use the same amps for each channel as i mentioned above. In your case, you have 2 different units powering each speaker so i'm not surprised that sound may differ. At higher volumes the HK525 may be running short on power IN COMPARISON TO the AVR8000, hence providing a difference in SPL (sound pressure levels) from the individual driver sections. Keep in mind this is only one hypothesis of what could be happening and why your sound is unappealing.
As for the Axiom M80s vs Polk, you can do a search on these forums for "Polk" and see what comes up. It has been mentioned in the past and their may be some comparitive reviews you would be interested in reading. What i can say is that Axiom speakers are an amazing product for their price. Their clarity is stunning.
Ultimately if you are looking for a truly deafening speaker though (say beyond 105dB at 10 feet or more), you will need to seek out a speaker with a larger cabinet design such as those oversized monstrous, ear piercing units used at concerts or dance clubs. Alternatively you can find many 'consumer' but boutique name speakers in many places. One that comes to mind are the big Tannoy
Kingdom or
Prestige series. These are ridiculously expensive though and often are WAY beyond what most home consumers need just to produce sound volume.
Hope some of that helps.
