Arm Upgrade Suggestion?

Sorry about the lack of a budget in the post. I think I can stretch to $2500.

I appreciate all the great suggestions!

Without budget, hard to tell. The Jelco is inexpensive due to mass production but rest assurred, it will see off your old SME. I prefer it over the SME IV. There are some third party mods available too.  Not sure how a 12" will fit a standard 124 armboard so check that first.

Dear Lew,
The TD124 has a vicious taste !
Fitting to it’s armboard, does not predict compatibility soundwise.

I do not understand, nor will I ever understand, why folks pose questions such as this.  ("What tonearm, what cartridge, what turntable, etc, etc".)  It is an open-ended invitation to confusion, or did you think that 40 eventual responders (or maybe more) were going to come up with one and only one response?  Suffice to say that there are a number of very fine 12-inch tonearms available, both vintage and current production.  You almost cannot go wrong with any one of them that (1) suits your cartridge as regards weight and compliance, (2) fits on your turntable, and (3) you can afford.  You know this as well as I do.
Sorry for being grumpy.

0.---you have already reached it's limit with SME3012
So...
1.---swap the Delos --- for a London Reference
2.---sell the complete package --- for:
 
a). 
DP75/DA402/ART9 http://www.ebay.com/itm/DENON-record-player-system-DP-75M-DA-402-Free-Shipping-JUNK-Item-/2823031232...

http://www.ebay.com/itm/Audio-Technica-AT-ART9-0-5mV-High-Output-Dual-MC-Stereo-Cartridge-Japan-Made...

b).
DP80/DA308/ART2000 http://www.ebay.com/itm/DENON-DA-308-12-INCH-POLLICI-TONEARM-DYNAMIC-/272470793885?hash=item3f70865e...

http://www.ebay.com/itm/Audio-Technica-ART-2000-Rare-Limited-Edition-Reference-MC-Cartridge-/3320641...

c).
PL-70LII http://www.ebay.com/itm/PIONEER-PL-70L-Turntables-Record-Player-F-S-/262519118389?hash=item3d1f5bda3...

d).
PL-70PLL/ISOKINETIK  http://www.ebay.com/itm/Pioneer-Exclusive-PL-70-PLL-direct-driver-turntable-unit-for-pro-VG-cond-/29...

http://www.ebay.com/itm/ISOKINETIK-SILVER-MELODY-SILVER-WIRED-TONEARM-Hi-FI-CHOICE-5-STAR-RECOMMENDE...

Please Remember:
The TD124 does not accept modern perspectives on arm & cart.
It can not resemble a modern sound whatever your efforts.
All the hi-tech arms & carts can easily play better on many other TTs of much lower quality than your TD124.
At best, you may approach an acceptable outcome, but at the expense of the vintage glory.

All the best

If you want a 12" arm,SME makes four of them!
http://www.sme-audio.com/audiophile-tonearms-pickup-arms

VPI 3D arm.... you won't be sorry

Jelco 750L.