Any decent improvments left for my ES-1?

Yes, I'm still using my AR ES-1. Over the years it's been modified with parts and whatnot from Scillia and Sound of the Wood. New springs, platter, bearing, motor, plinth heavily damped with the SotW damping material. By and large it sounds pretty good. I'm using a Denon 103R which I will probably send to Soundsmith for a new tip and Ruby cantilever.

I'm sure the thing that's really holding me back is my old MMT arm (don't laugh!). Not a great arm in the history of arms. So I'm looking to replace it. Willing to buy used. I need some firsthand recommendations on a good arm that works really well with this table and low to medium compliance carts.

I will admit that if I took all the money I spent over the years on this table and had it now I could probably buy a nice used VPI or Clearaudio. But if I can improve something that's already had the investment made by finding a good arm I'd choose that route.

Oh, if I can get an arm with some means of azimuth and vertical height adjustment, all the better. Looking for used << $1K.
I have owned several different versions of this table all with different arm/cartridge setups. I had one with a Linn Ittok arm that was really nice sounding and one with a Morch UP-4 that was also quite good.

The ES-1 is still an excellent table and the MMT arm, IMHO is pretty decent also, but yes, you could do better.
Mofi, I think the ES-1, especially after all the mods, is not bad at all. Yes it needs a rigid platform or shelf mount, and it may be just a tad soft in the low end compared to a non-suspended design. But it does a really nice job of putting out what's on the disc. The MMT I've found over the years is (IMO) sort of neutral to the point of being a little dead. Or maybe obscuring of everything that may be on the disc. Not sure how to describe it. In any event I really do think it's the weakest part of my TT right now.
Hmm, should have done a search first, My memory is slipping - I asked this same question in 2005! But back then my ES-1 had not yet been modified.

It seems like the the AQ PT-9 and Rega RB300 are the usual suspects for this table. Wondering further if anyone can give me a sense of how they might differ in sound, whether one works better with one type of cart vs another. And whether there's anything within that << $1K range that might be a real improvement over these two standards.
the jelco arm, which is the equivalent of the above mentioned AQ PT9 would be a significant upgrade to the mmt. other than another cartridge at some point, you're there man.
I think going for the COMPLETE Merrill platter system would result in a higher level of detail retrieval from your lps. The whole system is: 1)upgraded drive shaft/spindle; 2)acrylic platter; 3)lead mat; 4)outer ring clamp and 5)record weight.

In my opinion, once you own the whole platter system (originally designed by George and later refined by Anthony) you will never be able to go to another table.

Dave, I did it piecemeal, but I have everything except the outer ring clamp. At the time that was the limit of my spending, given that I also did a motor upgrade and chassis damping - oh, and acrylic mouting board. I may try to track down a clamp this year to complete the package. So, yes, I do have the majority of that configuration (plus a little extra). So what arm do I put on this thing to give me the next level for less than a grand?
the jelco arm, which is the equivalent of the above mentioned AQ PT9 would be a significant upgrade to the mmt

Jaybo, which Jelco?
I just set up a highly modded ES1 with a modded rb300 (incognito, mitchell etc) and a riggle vta. I made a custom arm board which entailed modifying the acrylic sub chassis. I was skeptical about the vta until I heard it. The guy I did it for was just going on conversations he had with Pete. In short I dont know how this combo could be improved on and with the cartidges I listened on it (I hate to say it) sounded as good or better than my SME V . I think total investment was not much more than a grand.
Great. Did you do the stub/spindle too? Don't discount the improvement of this part over the mass produced shaft/spindle pridcued by AR in the 1000s. These are singly made of a much better grade stainless steel to much finer tolerances.

Rccc, that's great info. I've heard that the Rega may or may not be a good match for the 103R, though. Something about tonearm resonances. I wonder if an Origin-modded version with the slots would address that.

Dave, yes I got the spindle too.
It worked well with a 103d, I dont know how comparable the 2 are.
Just had a friend over for some listening. We spent most of the day listening to my digital front end (PDT3 + Boelen modded DSP7000 Mk.III) and things were fine. Went to analog toward the end of the day and we were pleased with the extension and articulation at both the low and high ends and the quietness of the music. But there was just no "life" to the music. He used to own a MMT on his Thorens and said the same thing I was thinking. The MMT is just a dead, dead, dead arm. I haven't heard one cart on that arm over the years that didn't come off as having the same sort of dead sound. He concurred with his experiences.

I gotta get rid of this thing.
The Riggle vta seemed to add liveliness to the modded rega I described above. This is the only experience Ive had with it but I was able to compare the RB 300 both with and without it. I am always very sceptical of claims like those made for the riggle but in this case the difference in performance was easily heard. We listened to a 103D a Grado platinum an Ortofon (I cant remember which) and a Stradivari. Im hesitant to recommend systems like this without having more experience with it but it did work well in this particular instance. Good luck with your system.