Sorry late response, seems I'm not getting notifications on the thread:/
Thanks guys for the info, great advice here! So the system was a Theta Casablanca ii going via balanced cables into the ML23.5. From there, I can't recall anything special about the cables though they seemed thicker than my bare 12awg I have, nothing crazy though. For testing, I brought an ipad and went from the headphone out to RCA of the Theta, max volume on the ipad. This setup was how I was able to repeatedly hear the buzzing from bass notes. I then brought a separate setup to try and isolate the issue, SMSL SU-8 to XTZ A2-300 with a direct usb connection from the ipad, same issue.
I spoke with Rob (nice guy!) and he told me to pull the tweeter to eliminate that, and the problem could still be heard but stopped when I put my finger gently on the surround of the midrange. I noticed the cone was a little loose. Rob feels it's the midrange in need of a rebuild, though also suggested rotating the driver as well.
I'll reach out to Bill tomorrow, he's very close to me and I should've thought about him. Can I swap the midrange drivers easily to ensure that the problem is isolated to the driver?
Regarding amp choices, I'm open to the 23.5 as I know it's already a good match. I see skyfi has a BAT VK500 right now for roughly the same price as the 23.5 as well. I would welcome any advice on how to find a good pre amp to pair with either. There's an Eagle 2 on ebay for 700, wow that's a lot cheaper than the other two. It's tricky for a noob like myself to have an intuitive sense of what amps work with such power hungry, low sensitivity speakers. Compared to amps, buying speakers seems easy!
