Preamp

Are you sure its the sp6 that's providing your warmth. Audio Research preamps are not known for warmth. 

Other equipment

vpi prime 21 turntable

sp6

accustat tnt200 amp

martin Logan vantage speakers

if sp6 is not providing warm then changing to so 20 should change warmth

any comments sp6 vs sp20

Well, that is a hard one. I am a long time Audio Research fan. My first was the SP-5… so solid state. I have had a ARC Phonostage since back then as well. Now all of my equipment is contemporary ARC Reference. 
 

While there has been a little divergence from driving straight down the line towards the perfect natural / musical sonic reproduction. In general each step has been towards more detailed and dynamic reproduction without loosing the midrange bloom and rhythm and pace. So each five to ten years represents significant advances in the overall presentation. These steps each towards better reproducing the real thing.  
 

I would find it hard to believe you would feel like you lost something by the move, but gained a huge jump towards bringing the real thing into your listening room. I am currently listening to Oregon… each instrument is suspended in space with perfect tone… fully fleshed out… 

Anyway, given the probable value of your SP6… I would get the SP20 with the expectation that you will either get rid of it later, or not. 

If you only could have 1 which 1?

any other people to comment?

while I have attention I wonder if anyone can suggest a better cartririge

i have a dyavector 20x2 low output moving coil now 

Alternate Cartridge: Preferred/Better are separate considerations.

Sound Characteristics (do your research) are preferred, not better.

Imaging, is better, not preferred (good, better, best) and is vitally important. Imaging is improved by wider separation and tighter channel balance. The difference is easily recognizable.

Regardless of Phono EQ, Preamp, Amp: start with better imaging out of the cartridge. Change your Phono Stage, it will be the same: (preferred, not better) In any case, it receives better wider/tighter signal

your 20x2 produces 25db of stereo separation (mediocre), and 1db channel balance (good but 0.5db better)

Cantilever: 20x2 is solid aluminum rod. I never heard that. Stiffer material is better, in my experience: especially tighter bass. Boron, and Crystals are Stiffer than aluminum. I get Boron, not just a bit less money than crystals, but less likely to shatter a boron cantilever than crystals. IF you can afford them, and be very careful with it, then go for a crystal cantilever. I have a Sumiko Talisman S with Sapphire Cantilever, more/tighter bass?

We can convince ourselves of anything, but it seems so. I use two wonderful LP’s, ’Cecil McBee – Alternate Spaces’, (India Navigation – IN-1043) and ’The Bass’ (3lp set) (Impulse ASY-9284-3), an amazing collection of music/musicians/engineering.

All advanced stylus that are close to the shape of the cutter sound great to me. I have heard MicroLinear; MicroRidge; MicroLine; Line Contact; SAS; Shibata; Van den Hull; Geyer II. You can never do a direct comparison, each was on a different cartridge, so I would not let which advanced shape be the determinant.

All advanced stylus shapes MUST be properly aligned.

MC Moving Coil: Low or High Output. Low Output allows smaller/lighter coil, thus theoretically Low allows the advantages of MC, more refined signal. I have only heard Low Output thru my Step Up Transformer.

MI, Moving Iron: I have never heard one, highly respected by many.