Hi,
I already used a Duo, and Trio after that. I understand why do you want to improve LF. At that time, I had used a REL Studio III to make up the low freq. I think REL sub can help you to change the sound. REL is powerful and the price is not much expensive. And about the result of REL-Duo/Trio weding, I sold all Avantgardes and build my own horn speakers by using two pairs of TAD 4100 and 2100, and the REL.