Glad to hear you figured out the rumble problem. Having the sub too close will definitely create issues for your turntable.
I have never tried the phono output in my AV7704, it's possible that it's better than I think it is, but when comparing the phono out in previous pre-pros I've used to a decent outboard phono stage, there is no comparison.
An outboard phono stage should give you an improvement in sound quality and many of them will give you different loading and gain settings so that you can customize the settings to work best with the cartridge you're using.
I have never tried the phono output in my AV7704, it's possible that it's better than I think it is, but when comparing the phono out in previous pre-pros I've used to a decent outboard phono stage, there is no comparison.
An outboard phono stage should give you an improvement in sound quality and many of them will give you different loading and gain settings so that you can customize the settings to work best with the cartridge you're using.