Oregonpapa,
Based on what you say about the sound, I wouldn't worry about it.
I don't know about your record collection. You said you have tons of mono. I have quite a few old jazz mono LPs (< ton), and every one is a microgroove. Most are re-issues, but even one or two oldies are microgroove. For these records your carts will do a better job of extracting detail/nuance than an inexpensive mono cart. This is especially true if your records are in good shape.
Some of my re-issues are Japanese pressings and most are in great shape. Although the presentation with the MONO3 is nicer, I miss the resolution. It's sort of like using a stock 103 instead of your AT. It might sound nice, musical, etc, but .....
The noise reduction on some old monos is substantial and if you have original wide groove pressings you might need a 1 mil stylus anyway. I found an old copy of Mozart Requiem Mass in the basement. This was so noisy I couldn't listen to it. I'll check it out and let you know. It might be wide groove and I'm not expecting much.
You said your mono records sound good and you have a mono switch. That goes a long way. It would cost a ton of money to set up mono approaching the same resolution as your ART9/OC9, for a somewhat nicer presentation? Enjoy what you have.
Regards,