I'm very familiar with Axiom's speakers, and while I frequently recommend them, I'd have to steer you away from the M80v3 towers in your particular case. They require a fair bit of "breathing room" in order to sound their best. You'd pretty much end up with a very similar situation to what you're already facing with your Def Tech towers where placement doesn't allow them to perform their best
Given that you need speakers that can be placed quite close to walls, but still produce high output with low distortion, I would favor a sealed tower, with a front ported design being a possible second choice.
If you don't care too much about fancy looks (and since you were happy with Def Tech towers, I have to assume that plain, black boxes are ok by you
), then I have two good possibilities:
First would be Emotiva's speakers, with the ERT-8.3 being the tower option. What I like most about Emotiva's speakers is that they are essentially designed just like professional monitors. They allow for tweeter and mid-bass adjustment, which can be a huge benefit in less than ideal placement. They are sealed. And they have a somewhat controlled dispersion due to the driver arrangement.
Some things to consider though - if you are always going to be using a subwoofer, then the ERT-8.3 tower doesn't really make the most financial sense. You would be paying a fair bit extra for bass extension that you don't actually need and wouldn't really use! Instead, you could purchase three of the very high output large bookshelf ERM-6.3 speakers and glean a perfectly matched front soundstage while keeping the price a bit lower.
I LOVE the design of the ERM-6.3 large bookshelf speakers. They have a perfect 2nd order roll-off at 80Hz, which makes them an IDEAL match for any receiver or pre-pro that uses the THX cross-over scheme - which your Onkyo receiver does! Set the cross-over to the standard 80Hz point and you'll get a perfect blending of the receiver's 2nd order 80Hz roll-off combined with the natural 2nd order slope of the ERM-6.3 speakers to create the intended 4th order slope that THX is aimed to create!
Emotiva's ERD-1 surround speakers are also a favorite of mine. Their di-pole mode is extremely effective at recreating the surround field of a commercial movie theater. They are very affordable, small enough to easily mount on the wall at the desired 2-3 feet above seated ear level, and they too have the ideal 2nd order 80Hz roll-off.
And if the ERM-6.3 are too large, the ERM-6.2, while very boxy-looking, still perform like champs and have all the adjustability and ideal roll-off of the bigger ERM-6.3 speakers.
One other consideration with the Emotiva speakers though is that they really beg for powerful amplification. They are 4 ohm speakers and they can absorb lots of power, so they are a bit of a tougher load. This isn't surprising, since Emotiva is first and foremost an amplifier manufacturer
But you wanted a new 5-channel amp anyway, right? Emotiva's a great, great choice for that too.
The other choice that jumps to mind would be SVSound. The STS-02 towers are, again, a sealed design with somewhat controlled dispersion due to the driver arrangement. These play well in smaller rooms and with less than ideal placement, but they lack the adjustment options of the Emotiva speakers. They are physically a bit smaller though and they are also an easier load on any given receiver. The price is extremely affordable and SVSound also has a complete lineup of matching bookshelf, center and bi-pole surround speakers in the -02 series. So filling out your 5.1 system would be no problem in the future.
Again, I'd have to recommend staying away from rear ported speakers. With placement being close to walls, sealed or front ported is the way to go, with my preference leaning strongly towards sealed. If you can tolerate the plain, black, boxy looks and can satisfy the power requirements, then I think the Emotiva speakers would be my top recommendation. They have all of the characteristics that you need in your new space
