I see, yes, but that should serve as an important lesson – the soundstage is not real in the same sense as actual tones, bass, treble, whatever. Soundstage is a perception that’s based on many factors, and here’s a challenge for you: You should be able to find some music tracks that have better soundstage on one headphone, and other tracks that will be better on the other headphone. Most of the time it will be just one way, but when a closed headphone beats an open headphone, I expect the open headphone will still show an advantage on some tracks. Your hearing perception could be tricked by simple things like a recess or emphasis in certain frequency ranges, or even phase shift when more than one driver is in the cup.
That amp, if in good working condition, has 100 times the potential sound quality that those 2 headphones can play. You could improve the sound somewhat with a careful EQ, since the Marantz will have power reserve to spare. But I’d try to find a better headphone, and there are some bargains out there. If I were suggesting an ortho, I’d say get the lower price Mad Dog, which will give you great audiophile sound with the Marantz.
Music keeps me energized all day (and into the night) at work — 70% electronica/dance/DNB, 20% rock, 5% hip hip and 5% other/classical — but I’m tired of low-quality sound and I’m ready to put my money where my ears are.  I want to buy a USB DAC + Headphone amp, buy headphones (or, per your recommendation, to buy 2 pair) to complement the amp and my choice of music, and get great desktop sound for around $350.

Be sure to assess the build quality of your prospective headphones. Some earbuds and portable devices are relatively fragile, for instance. If the headphones fold up for easy storage, are the hinges robust, or will they fall apart in a month or two? Don't forget to consider that the earpads and earbuds will get extensive wear and tear over the life of the headphones.
Sensitivity is a measure of how effectively an earpiece converts an incoming electrical signal into an audible sound. It thus indicates how loud the headphones are for a given electrical drive level. It can be measured in decibels of sound pressure level per milliwatt (dB (SPL)/mW) or decibels of sound pressure level per volt (dB (SPL) / V).[12] Unfortunately, both definitions are widely used, often interchangeably. As the output voltage (but not power) of a headphone amplifier is essentially constant for most common headphones, dB/mW is often more useful if converted into dB/V using Ohm's law:
Cable dressing and length: Most stereo headphones have just one cable, usually attached to the left earpiece (sometimes called single-sided cabling). Some models -- and all earbuds -- use a Y-cable that connects to both earpieces (double-sided). The actual cable plug, meanwhile, is usually one of two designs: a straight I-plug or an angled L-plug; the latter may be useful if your portable player has a side- or bottom-mounted headphone jack.

Sealed models are ideal for private listening, where you don't want the sound to be heard by other people. Open headphones -- such as foam earpad models and many sports designs -- are acoustically transparent and allow outside sound to be heard by the headphone wearer, and a good deal of the headphones' sound will be audible to anyone near the listener.

In-ear headphones are like earbuds but are an upgrade to them. These headphones fit into the wearer's ear canal and stay in place with the use of foam or rubber tips. These tips are available in custom sizes so they can fit each individual wearer. When a proper fit's achieved, in-ear headphones reduce outside noise and deliver sound quality at about the same level as over-ear and on-ear headphones.