About $400 all-in

OK, so lastly...if I do end up opting for the Senns, with the 300 ohm impedance, I'd be best off getting an amp that is rated for up to 300 ohm, and if I get one that is only recommended up to 150, it would be a stretch for the amp's power output?  Otherwise it looks like many other headphones...

Thanks for the tech data.  How does impedance factor into it?  For example...Sennheiser says the HD600 has a 300 ohm impedance, and the Fiio A3 says it's recommended for headphones from 16-150 ohms.  How does that affect performance?  Do you have to get the A5 amp, which is recommended up to 300...

OK...silly question...what do you mean by "efficient?"

Thanks for the heads-up.  I haven't heard of them before.  Will def check out the info.

I have read the thing about the Senns being hard to drive on here as well, that's why I thought an amp might help.  I could be wrong though.   Thanks for the heads up on the ATs.   My total budget is about $400.  I guess I "plan" to continue to use it the same way I have thus far... .mp3s...

So, I've been looking on here for a while trying to make up my mind and finally decided to just go ahead and start the 10 millionth thread like this to get some advice.   A little idea of my novice status:  I currently have a 4-year-old set of Sennheiser HD 439s, some Sennheiser PX 100iis for...