New hd598 out of E7/E9, popping sound distortion at random intervals

I love the 598s, out of the box they were okay but the more I used them they really opened up to the soundstage that people speak of.  Anyway, I didn't experience any problems right away, but now I'm getting a crackle / pop sound randomly and at any volume.  I'm guessing / hoping it's some distortion rather than a faulty piece of equipment.  Before now I never used an amp as my previous cans were low imped. k540s.  These I know are still only 50ohms but I got the e7/e9 combo for the heck of it as an intro rig and for use with future higher impedance cans.

Anyway I'm sure it has to do with the e9, because I haven't noticed it when using pc soundcard / e7 with ipod (for testing purposes don't worry lol).  Is it just too much for these?  I know they're sensitive but I thought I could use it as long as I didn't crank the volume / have on high gain.

I have no EQ

I keep e9 volume at 50 - 70% and manage music volume in Foobar.  Never turn it up to un-Godly heights.

On my pc, I have a sb titanium series (not sure of exact model), and it seems to be decent for driving these.  I kinda wish I hadn't bought the e9 at all.  Is it faulty or just not at all meant for use with the 598?  I know it's overkill but the e7 seems to be too little for me.  Anyway I'd like any input.  Thinking of just selling the damn thing now as it's mostly been a hindrance.

Yeah I get the same problem with my others, and the 598s are fine from other setups.  Guess I can just switch out the E9.  I would like to keep it for future cans like I said, though I'm considering selling both E7/E9 and just put it towards a better amp / dac not made in China low end stuff.  Just got them for the affordability as an intro rig for amping.

Duly noted.  I'll look into replacing it then.  Thanks a bunch.