subwoofers and panels don't mix

Eldartford is correct in saying you need large diameter short excursion cones.Most mainstream subs using long excursion drivers with heavy rubber surrounds are going to sound slow and smeared.They are just wrong.
I can recommend the RCF L15 /554k 15 inch woofer[superb quality Italian pro speakers] as a great driver for using with panel speakers.Put these in a 100 litre box and port to RCF recommendations and drive with any half decent subwoofer amp and you will get that extra octave of bass[33-66 hz] that many panels lack.And it will integrate superbly and be fast sounding.Also this drivers high sensitivity means something like a 100 watt sub amp is more than adequate.

Audiokinesis,
Good to hear about your plans.You are right about the surrounds too.I have always preferred cloth surrounds to rubber.
I understand that many of the RCF woofers are now back in production.
I have mounted the L15/554k in a coffee table style box[firing down].This seems to sound best in the middle of the room.The gap at the bottom is about 100mm[4 inches].
This sounds superb with ER Audio stats as well as full range point source type speakers like Lowthers etc.
Many people swear by a high pass filter on the main speakers but I have found this is not really necessary unless they go below about 50hz-which most panels struggle to do.
Good luck.There is a real need out there for this type of low bass speakers that mainstream subs fail to address.

JT