Looking for ideas on Lyra Delos - muddy middle

I realize I'm opening up any number of Pandora's boxes here, but given that the cartridge designer is a regular visitor/contributor, I'm hoping to get some great feedback in addition to the great feedback I'll be getting from all of you.

Here's the dilemma - I recently bought a used Lyra Delos (claimed to have less than 400 hours on it) cartridge to mount on my current rig.  Details on that setup later in this post.  After some serious VTA adjustment/validation, Azimuth adjustment/validation and verifying the rake angle is accurate, I get what I think is the best sound I can get out of the setup of the cartridge. And, with 200ohms loading, I've sanded off the highs while keeping the lows and everything there is in check - detailed and musical.  Yet on a number of recordings (Rock mostly, but some jazz and psych with large transient swings or lots of instrumentation in the moment) the 'music in the middle' sounds muddy and congested especially on faster passages - almost smearing like I'd expect on the highs except the highs (for the most part) are detailed and cymbal decay, for example sounds right and not smeared or slushy.

Great channel separation, great soundstage, just a clogged middle.  Am I missing something obvious?  I feel like I've been back and forth and over and over and I'm just not finding anything.  I haven't gone back to my previous cartridge to check if it's something else in the signal path, but that would be the next thing if there isn't anything that comes from this discussion.  And, of course, if I go back and find that it's not the case with the old cartridge, then what?

My reference recordings to test are:

Arne Domerus - Jazz at the Pawnshop (German Pressing)
Beach Boys - Pet Sounds (Mono Reissue)
Dead Can Dance - Into the Labyrinth (MoFi Master Reecording)
Deep Purple - The Book of Taliesyn (EMI/Harvest)

and yes, the records are cleaned before playing.  


My rig:

Table: Thorens TD-124 Mk I with Mk II upgraded motor supports, custom dual-tonearm plinth
Cartridge: Lyra Delos (used, but no rebuild to my knowledge) w/Boron Cantilever/Microridge stylus
Old Reference Cartridge: ParadoxPulse modified Denon103R w/Sapphire cantilever/Microridge stylus

Tonearm: Esoteric Sound S320 Mk II
*Type: Static balance, tubular, S-shaped, with removable headshell
*Effective Length: 9.02 inches (229 mm)
*Effective Mass: 13.4 Grams
*Resonance: 8.5 Hz
*Spindle to Pivot Distance: 8.43inches (214 mm)
*Mounting Hole: 1-3/16 inches
*Arm mounting pillar diameter: 0.703 inches
*Overhang: 0.59 inches (15 mm)
*Offset Angle: 22 degrees
*Tracking Error: +1.9/ -1.1 degrees
*Bearings: Steel ball bearings
*Material: Aluminum, Magnesium
*Dampening: Rubber
*Finish: Matte Black
*Horizontal and Vertical Sensitivity: 20 mg
*Height Adjustment: 1.5 to 2.4 inches (38 - 60 mm)
*Tracking Force Calibration Range: 0 to 3 gm (for more force, turn
  weight past "0")
*Counterweight: 140 gms
*Anti Skating Range: 0 to 3 gm
*Cartridge Weight Range: 4.0 to 12 gms
*Headshell: Magnesium with oxygen-free copper Litz wire, adjustable azimuth

 Phono Preamp - Paradox Pulse Phono 70 w/200ohm loading

Headphone Amp - SRM-007I w/Mullard/Brimar tubes (6gc7)
Headphones - Stax Lambda Nova Signature (Serial - S00966)


The only thing I can think of is to further tweak cartridge loading, but that may cross the line of diminishing returns.

Thoughts?  

Thank you in advance for any advice you can give.