To be clear, I've never listened to an HD 600. Reviews I've read suggest that HD 600 has more output very low, sub 40 Hz, and HD 650 has more output at 60 - 100 Hz. Depending on the music, HD 650 could easily be putting out more total bass.

I have the HD 6XX and the DT 880 250 ohm. They sound pretty different to each other. HD 6XX - excellent mids, voices and instruments sound really lifelike. The bass rolls off compared to many headphones, but the midbass is somewhat elevated, say around 50 - 100 Hz. Just enough to slightly...

If 'in ear' means IEM (earpiece actually goes partly into ear canal), might you be OK with an ear bud design? (think the standard Apple buds that come with every phone). There are quite a few bluetooth ear buds out there that have a hook over the ear or within the outer ear. Some are one piece...

That's correct...

Plantronics claims 6, which is exaggerated, but I definitely get 4+.

I have no idea if these are available in the EU or not, but I've recommended several times the Plantronics Backbeat Fit. Around $60 at Costco here. I have destroyed a variety of headphones from sweat, but these have been reliable for at least 2 years now. They are a behind-the-neck style with...

I have the DT880. Agree that they're sturdy & comfortable. Bass is excellent for me - no 100 Hz bump, and really good extension. But bass lovers (you know who you are) generally find them lightweight. I like the mids as well, good transition from the bass, vocals (male especially) are rendered...

Sorry you got ignored first time around, but those are what I would have recommended. I have a pair that have been going strong for almost 3 years now, first running headphones I've ever had that didn't crap out in < 1 year. I'm pretty sure I've recommended these in the past, but it's a big site...

Earbuds have the lowest blockage of outside sounds, so why wouldn't any old cheap pair of earbuds work?

Yeah, you could use a dual RCA plugs to 3.5mm plug Y cable to plug into the microphone input on a phone or laptop or AVR, etc and play back through that device's internal amplifier

Both the Monoprice & Parts Express 12" subs have high level inputs

Starting with the amp, Parts Express has a bunch of inexpensive ones that would be suitable. Here are 2 examples. The first one is stereo amp only, lots of power, all positive reviews. The second one is a little less power, but has a subwoofer output if you ever want that, and Bluetooth. Brand...

There are 2 ways you could do this. 1) as you describe, an amp & a pair of passive speakers. connect amp to laptop from headphone jack, connect speakers to amp with speaker wire. there are so, so many choices for amps & speakers, but you can easily get pretty good sound for your $700. where do...

Then I think the answers to your 2 original questions are: 1) Modi MB is not significantly cleaner than Modi 2 using USB input 2) A clean up device wouldn't seem to be of any help in your setup since you don't perceive a difference between USB and SPDIF input from your soundcard

He has the Multibit, so he has SPDIF. Cheapest solution by far.