What determines the volume of a Turntable?It's tempting to say, "Dunk it in a bathtub like Archimedes", but I won't. ;-)
1. Cartridge output
2. Phono stage gain
3. Line stage gain
4. Amplifier gain
5. Speaker sensitivity
These are the major factors affecting SPL's in your room when playing a record. Of course #s 3-5 affect SPL's when playing any source component.
...how do I know which Phono Pre Amp to buy with my Turntable?The idea is to match cartridge output and phono stage gain. The goal is to send a signal to your line stage that's similar in level (voltage) to line level sources like your CDP.
KABUSA's phono preamp gain computer is a good resource for calculating suitable phono stage gain for a given cartridge output, or vice-versa. Plug in the parameter you know, in your case the gain of the NAD phono stage, and it will tell you approximately what output level cartridge you need. Understand there's some latitude here, but you don't want to be wildly off.