An 8" Lowther in a sealed enclosure would be ideal.
I think the bigger problem is finding a "fast subwoofer." Subwoofer by definition operates below a woofer, and would operate at 40Hz or below. Most/all commercial subwoofers claim to operate from a 100Hz down and that is probably stretching it a bit for music reproduction. You would need a direct firing midbass/bass enclosure to ideally mate with any speaker crossed over at 300Hz. Unfortunately, I do not know of any high quality commercially available midbass/bass enclosures. If you are into DIY there are many enclosures that you could make (check the vintage Altec website for starters/ideas).
I think the bigger problem is finding a "fast subwoofer." Subwoofer by definition operates below a woofer, and would operate at 40Hz or below. Most/all commercial subwoofers claim to operate from a 100Hz down and that is probably stretching it a bit for music reproduction. You would need a direct firing midbass/bass enclosure to ideally mate with any speaker crossed over at 300Hz. Unfortunately, I do not know of any high quality commercially available midbass/bass enclosures. If you are into DIY there are many enclosures that you could make (check the vintage Altec website for starters/ideas).