I see, thanks. Then
this opinion baffles me, since the poster seemed to praise Dynaco QD-2 center so much.
I just need a center speaker to anchor a stable center image when sitting off-center. DPL (and DPL II) center is too dominant; it tends to collapse the stereo image and narrow the sound field.
This post, however, praises QD-2, saying that the sound stage is larger and stereo separation is better.
I can understand a change in sound stage/imaging and stereo separation in the context of changes due to rear ambience, but don't quite see how the Dynaco's center channel L+R could change those attributes appreciably (at least, in the direction claimed). Deciding that such changes are for the better is a whole different can o' worms. As you stated your goal, center-fill, does make sense.
You can try the hook-up without using the Dynaco. The basic Hafler hookup has one speaker connected to an amp's L+ and R+, for rear ambience (which is L-R, Left Minus Right). If two speakers are used for rear, one speaker's positive input is connected to amp L+ while the other speaker's positive is connected to amp R+, then the negative inputs of both speakers are connected to one another. For rear channel ambience no connections are made to an amp's negative speaker outputs.
For center channel, the hookup is the same as the two-speaker example, except that you tie the two rear speaker's negative leads at the positive post of a third (center) speaker, then tie the center's negative post to the amp's negative outputs. Note that whether you do the connection with wire or let Dynaco do it for you in a black box, you still need an amp with common ground between channels (or risk amp fatality). Dynaco adds a passive volume control (which you can add to your own circuit if needed) but without a VC you get full output (ie, no attenuation), which I'd bet dollars to donuts still won't be sufficient for your purpose.
The Dynaco was panned by critics, who also noted center channel shortcomings. Works better for ambience, but the effects are very recording-dependent. Since Dolby Pro Logic implemented the matrixing of rear channel as L-R (aka the 'difference' channel, or signal), Dynaco brought the old Hafler ambience circuit back with a center twist as a poor man's HT. YMMV, as always.
FURTHERMORE:
I also use a center speaker in my main rig. In my case, it is to provide dialogue clarity for video, due to getting blown off the sofa during action scenes for videos with very low dialogue output in quiet scenes. Even though I knew it was doomed for failure because of the huge difference in speaker sensitivity, I had an old Dynaco in the closet, so I tried it. Phooey. I've never found the center to have adequate output for any mix of speakers I've tried, but it had been twenty years since the last time I'd attempted this... and there it sat in the closet... and there it again sits.
I wound up feeding the record output of my Emotiva to a second preamp (NuForce). After the Nuforce I Y-d the two channels together on the way to a mono amp (Amphony 200, smaller than a deck of cards) for L+R to a single Minimus 7. For music, I just turn the NuForce off, or, optionally, down. This arrangement also allows me to use just the Minimus 7 and subwoofer for TV while letting my tube amp rest now and then. The NuForce acts as a buffer so that my Emotiva's output retains the stereo signal.
