How do I tell if it's my receiver or towers that are clipping?

I'm building a new living room theater setup and got enough parts in to do a 2.1 test. I've got an Onkyo TX-NR595 hooked up to a pair of Infinity Primus P363 towers (and center, but was turned off), and a pair of SVS PB-1000 Pro's. I got my basic levels set and played a few test songs like Dream Theater's A Nightmare to Remember because it has some deep bass/rumble with the thunder in the intro, but also good punch when the band comes in.

The subs shake the floor, but I noticed that the towers were starting to distort pretty decently before I expected them to. I originally had the crossover at 100Hz just as a starting point and brought it up to 180Hz on the receiver, but it didn't seem to make a difference. I was streaming via bluetooth from my laptop and I was upping the volume at the receiver, so it wasn't clipping on the input. I've got some proper quality wires running to the towers, so I believe the only thing left is either speaker or amp distortion. Thoughts would be appreciated!

Slightly different issue, but I also noticed, while playing videos, that my sound was delayed by ~2 second (vs the video on screen) but only when connected via BT to the Onkyo (though I haven't tried other input methods). My Dayton Audio DTA-2.1BT2 amp plays right on time using bluetooth, so not sure what's with the Onkyo sound delay...?