best cartridge for less than $700 for AQ PT6 arm?

I have used with success, such Cartridges as the Benz Glider, the Benz Ruby 3, and lastly, the ZYX Airy 3X SB on both an AQ-PT-8 Arm (Same Arm as yours), and the AQ-PT-9 Arm.

You make no mention of your Phono-Stage, and what adjustability is has? Or Phono Cable, which can also offer good sonic improvements with this Tonearm.

If just a basic MM Phono Stage, you might be better served with another very decent MM, such as the Audio Technica AT-150MLX (about $350), and a Phono Cable upgrade?

Perhaps more input from you will be needed, for us folks to make a better assessment? Mark

I nominate the Ortofon 2M Black. Should be a good effective mass/compliance match, and the reviews indicate that it provides a sophisticated, full-bodied, hi-res presentation well within your budget. Also cheaper to own than an equivalently priced MC.

It's what I'd get if I upgraded.

I agree with Viridian. The lower-end Dynavector cartridges have good speed and detail for the price. If you're looking for something warmer, the Grado Sonata can usually be found for around $325 as a barely used demo.