Group Delay and DSP delay are two entirely different things.How do you know about the low-end distortion and group delay? Is this something that you can back up with measurements? I have heard that the DSP in some (mainly the new 'pro' series causes delay.
You have a hard situation... Australia is a rough place for good Bass, it seems.
I would have a very difficult time looking at any sub under $1000 as capable of delivering clean and linear Bass output, and that's here in the US. Again, Infrasonic output is an absolute must in my book.