I have used both identical and different subs in my system and things sounded great with either setup. Geddes, an expert on subwoofers also feel thats non identical subs are fine. I have also had just as much trouble blending identical subs and the non-identical subs with the system. To be truthful, the identical subs were harder to get right in my experience. For different subs, use level matching and for identical subs, use gain matching.
Klipsch RF 7 based HT 7.4, Pioneer SC 35, Acurus Five 200 amp, Chase SS 18.2(2), VS 18.1(2), Samsung BDP F 7500, Asus/My Book Live HPC 4 TB
Yaquin VK 2100 amp, McIntosh XR 5 speakers, Samsung BDP F 7500