You can alter the port tuning on the Devon quite easily, to raise or lower the system resonance.
The 2528 driver will accept the SRM 10B paper cone. This is a conversion I'm planning, as I have a pair of Ascots knocking about. The drive units will hopefully end up in transmission line cabinets.
I don't see why a Cheviot sized box would be unacceptable for the HPD 315 units. Surely this would take up no more room than Devons on stands?
I assume you use stands? If not, that would most likely be the cause of your room matching issues.
Main system: MMs/ADCs/Low output MC's/One rare Japanese SUT/One scarce British phono stage/various tonearms/hefty Japanese DD TT and hefty Japanese BD TT and small British BD TT/Swiss Idler jobs. 4 CD players/2 jitter buster/2 DACs/Valve buffer. TVC, stepped attenuator or valve pre-amp/Class A SS power-amp or Class A EL34 valve monos or big Japanese (part Class A) integrated. Big and smaller dual concentric speakers and full range speakers, amongst others. And too much more to list!