Any of the Monitor Audio Platinum center 3-way speakers are outstanding. Make sure you get to a 3-way if you want the most performance.  2-way centers have limited dynamic range and narrow sweet spot.  Don't worry as much about -3db point. :)

PS - The Gold line is also good and less expensive.  Sorry I missed your price requirements.

@erik_squires The cheapo Gold go for TWICE my budget, sadly. 

How about Monolith by Monoprice THX-365C (3 way) or SVS Prime (3 way)?

Emotiva Airmotiv C2+ used under $400. Any good? 3 way (5 speaker).

I would not use the CC6 as it’s not in the same performance league. If you can stretch a bit, which you should as the center speaker is critical for good HT performance, here’s a nice HTM61 that’d be a much better fit.
https://www.usaudiomart.com/details/649949818-bampw-htm61-s2/

If you’d rather not buy used I think this SVS center is excellent and would likely blend well with your speakers. They offer a 45-day, risk-free trial with free shipping both ways.
https://www.svsound.com/products/prime-center

Hope this helps, and best of luck.

I'd go with SVS for two reasons:  1 - I know those woofers and they are very smooth. 2 - Better crossover points. 

Polk Audio LSiM706c experience/comparison?

@soix Difference S2 (black poly bass drivers?0 vs 'no S' (yellow driver cone, like my 804)? Found 600 SERIES HTM61 with yellow for under just over 400 shipped. 

I think the S2 is much better. Note the dedicated mid driver in the S2 is the similar yellow Kevlar driver as in your speakers, the tweeter is probably better and more refined, and the woofers will be less of an issue in terms of matching to your speakers than the all-important midrange driver (and the better tweeter). Also, note that in the S2 the tweeter is mounted directly above the midrange, which helps avoid the lobing that can change tonality for people sitting off center in an MTM design where only the tweeter is in the center between the mids. The S2 I mentioned is in great shape and basically the same price as the S1 you’ve found, and to me it’s a no brainer. Just my $0.02 FWIW.

@erik_squires "Better crossover points" what makes a crossover point 'better'? I can imagine 'right' (for the speaker/driver) and 'wrong'. But a 'good' one for one speaker may be the totally 'bad' for the next speaker. And for very obvious reason "no" crossover point would be best (high pass residing in receiver, cutting low frequency off for the sub). Some argue the human voice range should fall withing on path. But there are 4 way  speakers that could be used to proof the point wrong. 

Hey OP:

While the selection of crossover points has to do with the individual drivers, a higher crossover point here means the tweeter works less hard, and may be a good indicator of higher dynamic range.

So, given no chance to hear a speaker, and going only by spec, I would pick a higher crossover point.

Given what I've seen of available mid ranges and tweeters, I'd be more comfortable with a 3-way that crossed above 2kHz.

Best,

Erik

B&W HTM61 vs HTM61 S2: Both seem to use the yellow honeycomb mid driver (similar to my 804). Tweeter similar looking (?). Both are 3 way. S2 has 2 bass drivers vs normal with only 1. Will check lowest used market price but S2 seems to be quite a bit more expansive. Worth the extra money (think 30% more for example)?

HTM61 'normal':

Specifications

Type: 3 way, 3 driver center channel loudspeaker

Frequency Response: 30Hz to 50kHz

Recommended Amplifier: 30 to 150W

Crossover Frequency: 350, 4000Hz

Impedance: 8Ω

Sensitivity: 90dB

Bass: 1 x 165mm kevlar cone

Midrange: 1 x 150mm kevlar cone

Tweeter: 1 x 25mm aluminium dome

Enclosure: bass reflex

Finish: black ash, light oak, red cherry

Dimensions: 198 x 545 x 339mm

Weight: 15.4kg

HTM61 S2:

3-way vented-box system 1x ø25mm (1 in) aluminium dome high-frequency 1x ø100mm (4 in) woven Kevlar® cone FST™ midrange 2x ø165mm (6.5 in) aluminium cone bass -6dB at 42Hz and 50 kHz 50Hz - 22 kHz ±3dB on reference axis Within 2dB of reference response Horizontal: over 20º arc Vertical: over 60º arc 88dB spl (2.83V, 1m) 2nd and 3rd harmonics (90dB, 1m) <1% 95Hz - 22 kHz <0.5% 180Hz - 20 kHz 8V (minimum 3.0V) 400Hz, 4kHz 30W - 150W into 8Ω on unclipped programme 0.1V Height: 218mm (8.6 in) Width: 590mm (23.2 in) Depth: 304mm (12 in) cabinet, grille and terminals 17.2kg (37.8 lb)

If cheap enough, would this old boy work with my 804 in large room?

B&W CM Center 2

The CM Center 2 descriptions reads a lot like the B&W HTM61 S2 including dimensions and chassis

Update: I ordered a new SVS Prime Center and a used B&W HTM61 S2 (both about same price). Will run them both in my system to determine which one to keep.

System is an VERY large volume room (2 stories high, open on 2 sides to several room), serves as TV/Movie (5.1) watching, stereo audio, SACD 5.1 audio. 

Any predicitions? 

Great.  Please let us know how it works out if you could.  Make sure you let the SVS burn in before making judgements, and maybe ask them how much time they recommend.  It’s nice they give you a generous 45-day trial period.  I think you’re gonna be a very happy camper one way or the other.  

So, received the B&W HTM61 S2 and the SVS Prime. Both cost roughly the same (used B&W in very good shape, original box and packaging, etc, SVS brand new). 

Hooked up to my 5.1 and did some back/forth switching and listening. 

Then turned the sub off to focus more on the centers. Very obvious differences between the 2:

- SVS very 'sharp' highs

- SVS mids sound a bit 'compressed'

- B&W has more bass

- B&W overall sounds more 'harmonious', vocals have more weight and sound more natural. 

Important note (and I am not how much this effects the above): the SVS have only a few hours of burn in time. I will keep them running a bit longer on the 'small room' system, then redo the comarison (the B&W is used, so should be fully burnt in). 

If I had to pick one right now, hands down the B&W