Yup – vintage gear, Yamaha’s first-generation DD processor/multi-channel amplifier.
Indeed...I have some experience with Yamaha's stereo receivers (as well as Kenwood's) years ago, and connected to a graphic equalizer, the Yammy's sound was pretty dramtic -- I then fell into Onkyo surround products and have been a devotee ever since after hearing a friend of mine's Onkyo stereo power amp and then one of their stereo receivers in his system and both knocked me off my feet (for the money, of course...)
Can’t say I’ve ever seen any true DD mono discs; most I’ve seen are 2-channel, and they work as they should in DPL (i.e., sound from the mono speaker).
Yes -- you are right...99.9 percent of Mono-encoded discs come with a "2.0 Mono" track, not a true 1.0...but just recently, I rented THE OMEGA MAN and Warner actually equipped this disc with a TRUE Dolby 1.0 mono mix, which my receiver dropped into its MONO MOVIE mode, but everything just came from the center...
Let me explain a little better; in my receiver's LISTENING MODE PRESET menu for setting the listening mode presets for the different inputs, under DVD, I have "DIGITAL FORMAT: MONO" set to MONO MOVIE so these "real" 1.0 mono tracks drop into that mode -- I have chosen to have this the default mode for these real mono tracks because the other ones to choose from --when running these 1.0 mixes -- are all those odd DSPs I mentioned that don't sound all that good, like FULL MONO, MONO, STEREO, etc...so I have left the LISTENING MODE PRESET for Mono 1.0 tracks to MONO MOVIE...
Now, when the receiver sees a "2.0 MONO" track, it engages the PRO LOGIC II MOVIE mode as the default listening mode (as I set it) and it drops the entire track into the center channel -- I know a lot of people would argue that ANYTHING "2.0" should definitely be played out of at least two channels -- the main fronts -- but this sounds so weird to my ears. The mono track split between two front speakers has a weird "comb filtering" effect where I simply can't make out where the dialogue is coming from, so I avoid it and let the Pro Logic II Movie drop it all into the center channel...would you say this is preferred?
In my case, the Mono movie mode is only available with DPL anyway. Like yours, it adds some reverb to the other speakers, but it gets tiring after awhile.
Well, you know what's weird is that, as I said, this "Mono Movie" mode DOES NOT add any kind of "reverb" to my speakers at all -- I have tried this on several discs...but another interesting thing is that I read a review on another Onkyo receiver (let me see if I can find the link for you) and the reviewer said that according to his tests, this MONO MOVIE mode's reverb could only be heard on 2.0 MONO tracks, NOT the 1.0 mono ones...strange, huh? And I have the receiver set to default to MONO MOVIE for only the 1.0 tracks...for 2.0 Mono tracks, I have the receiver set to default to PRO LOGIC II MOVIE...
Does all this matter for mono anyway? Should everything just come from the center? Is that the INTENTION of mono?
Your Mono mode seems to work fine in DPL. That dropping it in the two front channels thing might be related to Dolby Digital encoding for a 2.0 disc. If that’s the case, then it is what it is, I guess.
Well, I think there's some confusion here with regard to how my receiver is working; let me see if I can explain it a bit better...
The Onkyo's MONO MODE is different from PRO LOGIC II -- see, MONO, FULL MONO and MONO MOVIE are three totally different DSPs to choose from in the receiver. Now, when the receiver senses a "2.0 MONO" track, it engages PRO LOGIC II MOVIE automatically (because of the "2.0" designation) and drops the whole track into the center -- but as far as dropping it into the two front channels, the only way I can do this is to enter the receiver's DSPs and select MONO (which mysteriously decides to send the mono track to both front mains) or perhaps select STEREO, which does the same thing...
So, should I just be leaving the 2.0 mono tracks dropped into the center channel? Like I said before, playing a mono track over the two front channels sounds really, really weird to me -- I look to do it so there is some "image spread" over the center, but it ends up sounding strange with a "comb filtering" effect that sounds kind of awful...
Like you said, the receiver might not be treating it as advertised.
Well, I said that about the MONO MOVIE mode -- the manual for my receiver says the MONO MOVIE mode is supposed to add reverb around the soundstage when running a mono track -- but I do not hear it.
Feel free to try the “Full Mono” mode, but I predict you’ll get tired of it before the first movie is finished. 
Indeed Wayne; I have tried this mode and it is just like ALL CHANNEL STEREO, as Onkyo calls it, putting the track through all channels at once -- it does indeed sound awful. There's something really strange about hearing dialoge from all channels, and the whole thing sounds "weak" and "wrong;" I forced it into this mode so there can be some kind of "surround" experience with mono tracks, but I just don't think it works, as the sound is all over the place...
What would be the point or application for using such a mode as FULL MONO? I can understand someone using ALL CHANNEL STEREO if they're piping in music for a party and speakers are around a room; but why would someone even want a film soundtrack coming from every speaker?
At any rate, you watch all your older discs that have mono tracks just from the center?
Thank You,
SM