Under seat subs are awful very poor sound.
Rather than loose my boot space I went for decent components up front (amped).
Did away with the standard rears and built an acoustic shelf,fitted good quality 6*9 in it and used the spare amp channels for these. Use them purely for bass notes.
Sounds better than an under seat anyway,car audio centre fitted and tested various options till I found what worked for me.. I know this doesn't really give you an answer,just trying to steer you away from the under seat things as they are really poor.