I have a pair of Tannoy Revolution R2 floorstanders - I’ve owned them from new but they are 20+ years old now and sort of superfluous but I’m also sort of attached to them. A 2.5 way design, rear ported with a conventional tweeter and 2 identical 5.25 inch drivers. The cabinet is a braced MDF construction with 18mm panels and a 25 mm front baffle + real wood veneer. Nominal impedance is 6 ohms but tests at the time suggested a dip to 4 ohms in the bass region - not surprising with a 0.5 way woofer configuration.
This generation of Tannoy Revolutions (R1 stand mount, R2 and R3 floorstanders) had a lot of input from Karl Heinz Fink (Fink Team, new Epos and also Naim Ovator AFAIK). The floorstanders had a reputation for being smooth and well balanced but lacking openness and punch/drive - I would certainly agree.
Anyway, I studied the crossover which is in two parts - a main 2 way board for tweeter and mid-bass driver + a second board for the 0.5 bass driver. The speaker has biwire terminals - the top set goes to the 2 way board, the bottom set feeds the 0.5 driver. I decided to remove the terminal links (speaker wire) and just feed the top section, leaving the bass woofer to effectively act as a passive radiator (PR) - which it certainly does.
The net effect is a brighter and clearer tonal balance (not surprisingly) but the bass response is more full than I expected - perhaps aided by the redundant bass driver acting as a PR. I tried it with the supplied foam port bungs both in place and removed and I preferred it without. The speaker certainly sounds quicker and more punchy which to me suggests that there was something strange about the original output level and/or phase response where the 0.5 driver was involved.
I have run these speakers in a passive bi-amplified configuration before which certainly helped as it reduced the low impedance load on each amp.
I now have a lot of experience of integrating active subwoofers and the active bass section of my OB system. This got me thinking that having control of the output level of the 0.5 section of a typical 2.5 way design could be useful for those who find 2.5 ways difficult to integrate in some rooms. Without modifying a speaker (and assuming it has biwire terminals) this could be done by running two integrated amps horizontally so that one is fed by the preouts of the other and one would be able to set the 0.5 volume level independently of the main amp. Alternatively, if using just one amp (as in most systems) you could use an L-pad attenuator in to the bass section. To make it clear, I am NOT suggesting that one goes into the speaker to and modifies the crossover!
As a design concept, 2.5 ways are a popular way of increasing bass output in narrow baffle speakers - a form of baffle step compensation (BSC) but the efficacy of such an approach varies very much according to the room and placement within the room.