My current 100W@8ohm Integra DRX 3.4, sounded congested, with a somewhat collapsed sound stage into my 4 ohm nominal Gallo speakers (efficiency is circa 86db/Wm) - listening level at MLP was 72db (using Radio Shack SPL meter) - playing stereo only.
Using it as a pre, and putting a Crown XLS2500 amp on to drive the same speakers cleaned up the congestion, brought back the full sound stage, and made the setup sound like I know it can and should... (it does 440W@8ohm and 1200W@2ohm) - listening volume was the same 72db.
The point is, when it comes to loads that drop below an amps design envelope, the audible results are highly unpredictable. Some, like my own Quad 606 amp, will handle it just fine, as long as you don't drive it too hard (it is specced as "stable into any load"... but can only put 90W into 2 ohm... that is equivalent to roughly 22W@8ohm... even though it is spec 135W@8ohm) - I know of no AVR that is rated for 2 ohm.
So when you take a 4 ohm rated AVR, like my own, and drive a nominally 4 ohm speaker, the results remain unpredictable... in my case, the tweeter drops down to 1.6ohm, and the amps get unhappy at that load level, and audibly start to misbehave... even though they are not being driven to very high levels.
Having said that, this is probably a relatively unusual "edge" case - but it pays to try to find out what the actual impedance/frequency behaviour of a set of speakers looks like, as it can predict such issues!