Jimmy-
Best wishes and sending good vibes to you and your colleagues at the station. You are to be commended for thinking of ways to improve the audience listening experience.
As I understand it, you are thinking about using a C-QAM exciter to drive the AM transmitter. Station will be broadcasting analog, and not HD Radio IBOC. Your idea is many HD Radio receiver AM sections will decode C-QAM stereo, and this opens up the audio high frequency response of the receiver. This will make the station sound better when heard on HD Radio receivers even though the station is not broadcasting an HD Radio IBOC signal.
I don't remember the technical details of C-QAM and not inclined to research it now. Posting because your might consider these points, some of which have already been touched on by The Big A and others:
1. Are there side effects of C-QAM encoding that could negatively impact audio for analog AM listeners? For example does it limit negative going modulation to a value some might consider a drawback? Does it apply high pass filtering to L-R audio? If you will broadcast mono audio yet encoded with C-QAM pilot, will the encoder do things to the mono audio? If you broadcast mono source material now (mostly) and this motivates you to use stereo source material more often, what are the music audience implications? Especially if you play '60s music. Playing the "right version" can be subjective. However if you start playing tacky re-makes and uninspiring remixes of hits to be in stereo, this may not be helpful.
2. Know your audience and your available audience. It is a demographic thing, and could be both are fine with a good sounding analog AM station. If they would like better audio, they might be more inclined to seek FM.
3. May not be as many HD Radio receivers that decode C-QAM in the hands of your audience and available audience as you might suppose. Sometimes radio folks get in a bubble.
4. Consider best way to spend money with goal of improving audience experience for the largest percentage of audience and available audience. You might get more bang for the buck with improvements to the analog audio processing, studio to transmitter link, and transmitter-antenna, especially if it is a directional antenna. Those are big ticket items, and the best answer may be to pass on them (even though you want to go for it) and do your best to acquire an FM translator. FM translator is giant cost of course , but it could resonate with audience hugely. I get what you said about translator coverage though, your audience may be dispersed over three counties and given incoming interference one translator is not likely to serve all of them.
5. Best money spent may not be on engineering. Since you are asking a nuanced audio oriented question about C-QAM and HD Radio receivers, would not be surprised if your station already sounds better than most other AM stations. Might instead increase time spent helping programming and on-air folks to the extent they want you to, and part of this is guiding money spent to things that help them.
So, you've got a lot to consider. Above all keep focusing on the audience experience and being financially wise.
Take care