What to upgrade first for better sound quality?

In my experience, the more money you spend, the better the results....given well respected name brands. As you get to the top of the heap, your money buys ever smaller increases in performance.

09-17-15: Stringreen
In my experience, the more money you spend, the better the results..

In my experience the more you think how to spend money for best value the better results you get no matter how much you spend.

Which component you should upgrade first is a hard question to answer because it depends upon several factors, including your assessment of the strenghts and weaknesses of each individual component, recognition of what you are trying to change, and finally your budget.

In general, I will say that if your goal is to make the most impactful change in your system the order would go like this, from most to least: speakers, analog front end (cartridge, tonearm, phono stage/SUT, turntable), preamp, digital front end and amplifier last.

I'm not saying that the components at the end of the list do not make a difference - they do. But complete character change of a system will only occur by replacing the final transducer or the source. IMO

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.