I posed the same question to Hartke and here was their response:
Riis, I replied to Hartke and mentioned the issues when set up in full-range. If they respond I'll post it here. Prior to this setup I had always used my 4x10 with no problems. I'm hoping the quirk when I had the balance all the way to one side for the 4x10 was due to the fact I still had the 1x15 hooked up even though I wasn't 'using' it.
Thanks for the interim follow-up. It is perplexing. I would hate to "cut the legs out" from your 4 X 10 at a xover freq of 250 hz when the cab itself is capable of handling content well-below 100 hz.
Riis