What to upgrade first for better sound quality?

You have a good cartridge, a good turntable, a good integrated amp,and good speakers. The one weak link that pops is the Mac's phono stage. Before you move to a low output MC cartridge, try to see what a quality phono stage would do with your current 2M Black, which is a great-tracking, great-sounding, high performance MM cart.

Maybe you could mail order a good phono stage and use their 30-day trial period to see how much it livens up your musical presentation. Or check with a local shop and see if they'll loan you a trade-in or demo model as proof of concept. Or maybe an audio buddy could slide one in to see. I suspect that lots of phono stages would outperform your Mac phono stage even if you keep the cart.

You might look into a Jolida JD9 mk II as they're reasonably priced for their performance, very good sounding, have multiple gain settings, and lots of settings for resistance for MC and capacitance for MM carts. The JD9 would handle any low, medium, or high output cartridge you might have now or in the future.

You might try to chase down a MAGI Phonomenal phono stage. It's MM, MI, and HOMC only, but it's a handwired tube unit for about a grand that's been favorably compared to the Manley Steelhead. I was lucky to have an audiobuddy who wanted to sell his; there aren't many out there. It brought my system to a new level, and I too have a high performing MM cartridge, the Audio Technica AT150MLX.

Or maybe contact Musical Sounds about there demo ASR Mini Basis Exclusive MK II;

http://www.musicalsounds.us/opbxspl.php

Anyhow, I would agree with Johnnyb53 that a better phono amp would deliver better results from the gear that you already have.

I would match phono stage to cartridge not the other way around. If I were you I would probably replace the Ortofon with Soundsmith Carmen or higher and see what happens. Then phono stage. Then speakers. Then amps. And cables.And power regenerator/conditioner.

Go with the herron and don't look back!

I suggest infrastructure.

First, clean up your records. I use 80 KHz ultrasonic cleaning, and it is like doubling the price of a major component (over my conventional record cleaner). Added benefit: extends stylus life. So before you spend big bucks on a wundercart, spend some money to protect it from groove grunge.

Second, clean up your power. Most power comes out of the wall looking like zombie apocalypse. Buy an isolation transformer from Plitron and put it in another room, because when it does its job it growls. For $1000 you get a significant sonic improvement (smoother) and, as above, protect your equipment from surges, DC, etc.