Turntable upgrade recommendations: SME vs AMG vs Technics vs other

You have some compelling options - VPI and Linn may be appropriate to add to the list. 
 

What are you looking for in regards to sound versus your P8?  And is P10 a consideration?

You are in a very good space. I suggest that for $2500 more you can avoid making a costly mistake and find audio Nirvana.

Take a listening holiday to NY and hear some turntables. Find what YOU like, not what someone else likes.

Also, suggest being clear about your needs as well as your likes. Do you need a suspended table? I don't; I live on bedrock, far from a highway. So I didn't pay for a suspension which I do not need.

What are your tastes? Do you like CD-like crystal brilliance (I hate it), or do you prefer a smooth, sweet, mellifluous journey? When tuning my air bearing turntable and tonearm, I found that most real improvements enhanced the latter.

How much hassle are you prepared for? How much quirkiness? If you are ready for even a modest amount of these, you can save a bunch of money AND improve a bunch on the sound.

Three basic technologies, each of which has adherents. Belt drive (technologically easiest, and IMO quietest), idler drive, and direct drive (every electronic twitch delivered faithfully to your ESL's). Get an air bearing if you can afford it - once exposed, there is no going back. Air in all 3 dimensions is friction-free and noise free, but costly.

Make sure that your choice of tonearm is compatible with your choice of cartridge. 

As for brands, consider some which do not advertise heavily, like Nottingham Analogue. Bit quirky but a wonderful sound - almost as good as my air bearing.

My system is all analogue, mostly DIY, with heavily modified Quad ESL's. Cartridge is Koetsu with diamond cantilever, tonearm is based on a Trans-Fi Terminator air bearing, heavily modified to accommodate the Koetsu.

Good luck!

I agree with adding brinkmann. I looked a a lot of turntables this past year and ended up with a Brinkmann Bardo. It's direct drive but with a magnetic system that is incredibly smooth. The bearing is amazing. The platter which is definitely massive spins for more than a full minute after turning it off. As the other user said, I have it on an HRS platform. It's an incredible piece of engineering but more than that, it sounds stupendous 

I have the AMG Viella Forte with a Lyra Atlas Lambda Mono and am in love with it

I had SME and have AMG and love it, I could not live without being able to adjust Azimuth...