Sam, there are ton of variables here that could be taking away from your enjoyment of your HT system.
To go into detail would be heresy on such a nice (and rare) day here so i'll just list them quickly instead. I'm dying for a brew out on the patio before the sparrows, i mean mosquitos, come out for the evening.
1) different sound from Axiom setup due to reference sound from older Infinity setup? e.g. How long have you had the Axiom setup? Was the Infinity setup typically more bass filled? Are the number of speakers the same? Different types? floorstanding vs. bookhself, 8 ohm vs. 6 ohm, what about speaker efficiencies?
2) different sound varies by dvd and its sound mix (the general lack of missing bass, was this tested with the Infinities side by side with the Axiom setup with the same movie and while calibrated to the same SPL?)
3) possible different sound due to 6 ohm QS8s dragging more power from the receiver compared to the old surrounds, hence a small power issue but would explain your necessity to turn up the volume to get 'more' from the Axioms
These are just a few possibilities but i do believe your Sony receiver should be fine.
You can always head over to a box store, pickup a nice receiver, try it at home (equalize the test to run at the same SPL) and see if it really sounds any different from the Sony. I would not expect alot, not unless that Sony unit was really built with plastic and straw.
