Stereo piano?

Given your equipment, I would not buy a mixer until you’re standing in the store and know exactly what problem you’re solving.

You already have:

• Yamaha P-125

• Arturia KeyLab 61 MK3

• Two Akai MPK Mini

• MacBook

• Two Peavey PV15PM speakers

• One XLR cable

The KeyLab and MPK Minis are primarily controllers, not sound sources. Their sounds come from the MacBook. The P-125 is the only keyboard here that makes sound by itself.

Minimum purchase today

I’d probably buy:

• Two good 1/4” cables (10–20 ft depending on your room)

• Possibly a second XLR cable

Then test everything.

Setup #1: Piano only (easiest)

• P-125 L/L+R output → Peavey #1

• P-125 R output → Peavey #2

This gives stereo piano immediately.

Setup #2: Computer sounds + controllers

If you want the Arturia and Akai to play virtual instruments on the MacBook:

• Arturia → USB → MacBook

• Akai(s) → USB → MacBook

• MacBook audio output → speakers

This is where an audio interface becomes useful.

If you want one purchase that solves most future problems

A small audio interface such as a used Scarlett 2i2 is often more useful than a small mixer for a MacBook-based setup.

Then:

• P-125 audio → interface inputs

• Arturia and Akai → USB → MacBook

• Interface outputs → left and right Peaveys

What I’d tell the guitar shop

Tell them:

“I have a Yamaha P-125, two powered Peavey PV15PM speakers, a MacBook, an Arturia KeyLab 61, and two Akai MPK Minis. I want stereo piano now and eventually computer audio through both speakers. What cables do I need, and should I get an audio interface instead of a mixer?”

That’s a very common setup, and they’ll know exactly what to show you.

Before buying a mixer, I’d first see whether the P-125 directly into the two Peaveys gives you everything you need for practicing and playing. Many people discover that’s already 90% of what they wanted.