Suggestions For Analog Upgrade - Tonearm or Phono Stage?

sbanks, nice system. Tough questions.

Nagra is known for making excellent equipment. I have to think you would need to spend serious money to better it.

IMHO the weakest parts of your system are the tonearm and cartridge.
The tonearm is stable not neutral balance. It is going to hunt for it's balance point and VTF will change with elevation. The vertical bearing is too high increasing the warble with changes in elevation. These two issues make a big difference in the solidity of the sound and image. In your price range you should look for a Kuzma 4 Point 9. This is a killer tonearm for the money and it will fit on the Nova. The other two that will fit are the Schroder CB and the Reed 2G but they are more expensive. The Tri Planar is an excellent arm but it will not fit. It's rear end gets in the way. You could modify the plinth cover by cutting away the section behind the arm board but this would not look so hot. The cover can always be replaced. I just read about the Helius Omega arm, another small British company. It looks like a good design and it looks like it would fit and it is reasonably priced. The final suggestion would be to look for a good used SME V. It fits perfect and it really is a very good arm. 

For a cartridge I would get either a Soundsmith Sussurro or Voice. They are exactly the same cartridge except the Voice has larger coils so it is higher output. The Sussurro is $2000 more expensive. I discussed this by email with Mr Ledermann asking him why. His response, "Different market." "That's all?" I replied. "Yup." I bought the Voice. The cantilever/stylus assembly is a work of art. The stylus looks very similar to the Gyger S, the quietest stylus I have ever used. Can't wait to hear it. 
IMHO the very best arm is the Schroder CB. Because the tonearm wire exits via the bottom of the spindle, under the tonearm board, it installs brilliantly on a Sota. It is neutral balance, the vertical bearing is at record level, the bearings have insanely low friction and the anti skating mechanism is magnetic, no friction. I has one wire from cartridge to phono stage. There are not intervening contacts or connections. It is a $5000.00 arm. I have not yet seen one used or off demo but neither have I looked hard. I think next I would go for the SME V as it also installs nicely on the Sota.  There are a lot of them on the used market. The Kuzma and Reed arms are excellent but the tonearm cables are a separate item and plug into the arms above the tonearm board. You can make it work but it won't be as slick. The Helius arm I have never used or even handled so you would be on your own there but it does look like a good design and the cable exits below.

Have you thought about subwoofers? You can turbocharge your Sound Labs that way. With music that is dynamic with a lot of bass it makes a very significant difference in overhead and distortion levels at volume. 

@sbank , yes, you are right. The Schroder Reference and Model 2 are interesting arms but not my cup of tea either. They are his earliest designs. I agree also on the Helius. Small companies have a habit of disappearing. However a well made arm should be essentially immortal. Value depends on what the mythologists do with it. The SME V does just fine without those adjustments. 

Updating the Audiomods arm will not change it's basic problems. You have to get the center of mass of the cartridge and the center of mass of the counterbalance in the same plane as the vertical bearing to achieve neutral balance. You can't get the vertical bearing down at record level without severely messing up your VTA. If you have not watched this yet please do.  It is a great primer on turntable and tonearm design. I only disagree with Villchur's  position on anti skating.    https://www.youtube.com/watch?v=1rgK0YMsJXM

Spencer, it takes a big leap of faith and you have to take a deep dive into digital territory but you can create an incredible system putting subs under Sound Labs. It is not a swarm system but it will require at least 4 individual subwoofers. If you are ever in the Boston area I'd be happy to demonstrate :-)

Listening to and using a tonearm are the final steps in evaluating an arm if you get the chance. Tonearms are straight forward mechanical devices. Most of the elements of a tonearm's design are clearly visible. 
My own approach involves a set of criteria which I think are essential in achieving the best performance. Once those criteria are met it becomes necessary to evaluate criteria which are not so obvious like the resonance characteristics of the arm tube, ease of use and quality of construction. Using and listening are the best ways to do this but unfortunately, many of us to not get the opportunity and have to go by the opinion of others.

With arms that are being modified by secondary parties you can not be sure of what you are getting. SME is a very technically sophisticated company. I doubt anyone is going to make a significant improvement in performance with one of their arms and is more likely to do the opposite. Why would I buy an arm that needs to be improved? There is nothing I can do to a Schroder CB to make it perform better.