Hi Bagwell359.
nice to hear from you.
The HIFIMANN HE500 is my favorite from Fang's inventory so far.
I find that most of his designes have serious flaws in the treble or the midrange, and in the case of the HE500, i think he got it mostly right.
Its just a very good sounding headphone that does not irritate you in some way, as most of his headphones seem to do.
The bass could be tighter, perhaps.
Really good Planar or Dynamic headphones, all seem to have a similar sound.....there is a clarity and a truth to them, regarding how they present their sonic imagery.
Regarding your questions..
The Quads have more depth and richness, while the HE500s have more "airy" openness and upper mids.
Its Elegance/Resolution vs Stark Reality/Straight Wire
I dont mod any of my headphones, as this is an endless circle.
I mod my Guitars and i mod my Nike's......
I will change pads or wires.....so, if that is "modding" them, then yes, i'll go that far.... but, i think you mean something more advanced/destructive when you speak about "modding".
Also, i argue that anything you put between the speaker and your ear is subtraction from the sound, that some call "modding".
But the fact is, if you put something between your headphone and your ear, you are not modding, you are in fact, blocking or restricting.
It like this.....if you have a set of speakers in your room, and you put earplugs in your ears, then you listen to your speakers and say......"yeah, the treble is better now, or the bass is worse".... = modding headphones with filters.
I dont support this.. and in fact, i feel that if you have to try to remove EXCESSIVE bass or treble to be able to <enjoy> your headphone, then your headphones are just not very good to begin with.....i believe, or at least they are not what are going to satisfy you.
Get something else.
My opinion is....if it does not sound its BEST as delivered from the factory, without inner screens and all this hyper modding of the frame jazz, then get another model that does come from the factory with its best sound built right in.