#1
I have played guitar for over 15 years and only using Marshall valve amps and the likes.

I can't play on high volumes with big amps anymore so I'm left with playing with headphones.

I got a Sonic Port VX about a year ago and play with the mobile POD app and Apple EarPods.
The sound is very disappointing, flat and boring.

What can I buy to get better quality sound when playing with headphones and a Mac/iPhone (if its possible).

I play mostly Metallica and my favorite sound is the sound on Master of Puppets.
#2
First try your current rig with something better than four buck ear buds. Take it to a store and try over-the-ear headphones in the $100 range (Sony 7506, AKG240, Sennheiser 280, etc.) and see if that makes a difference.

Find a used Pod XT bean and plug better headphones into it and see if that works better. A used XT bean will be around $80-90.
#3
+1 on the better headphones. All of my practice headphones are over the ear: better speakers, better fit, better sound.

You don't have to spend big $$$ on them, though: if you check out company outlets, refurbished headphones can be had discounted up to 90% off.
Sturgeon's 2nd Law, a.k.a. Sturgeon's Revelation: “Ninety percent of everything is crap.”

Why, yes, I am a lawyer- thanks for asking!

Log off and play yer guitar!

Strap on, tune up, rock out!
#4
You might also want to get an actual modeling device or amp that has a headphone jack. I love my Mac products, but there is something to be said for using a specialized tool as opposed to a multitasker.
Sturgeon's 2nd Law, a.k.a. Sturgeon's Revelation: “Ninety percent of everything is crap.”

Why, yes, I am a lawyer- thanks for asking!

Log off and play yer guitar!

Strap on, tune up, rock out!
#5
OK. Thanks for the advice. The Sennheiser HD 280 Pro is ordered.
I'll keep my eyes out for a used POD XT.