New to headphone amps --- I'm wondering .....

Lots of good points have been made above. I would particularly emphasize three of them, and add a fourth.

1)

02-10-14: Doggiehowser
The relative amount of power needed to drive cans (even something as difficult as a pair of Hifiman HE6) is a mere pittance compared to a regular pair of speakers so you don't have to pay for more power. Just a small amount of high quality ones.

To add some quantitative perspective, the maximum amount of power your 545s are rated to be able to handle is 0.05 watts. And in response to that maximum input power they will produce a very loud 99 db of volume. In comparison, a pair of box-type (non-planar) speakers rated at 90 db/1 watt/1 meter will require about 40 watts, or 800 times as much power, to produce the same volume at a typical listening distance of around 10 feet.

2)

02-15-14: Mulveling
In your case, by running from tape-out you're almost certainly creating an impedance mismatch ....

The tape outputs (as well as the main outputs) of your Exotik preamp are spec'd as having an output impedance of 330 ohms. That is about 10 times greater than the 32 ohm impedance of your headphones. For a line-level interface, ideally the load impedance should be at least 10 times greater than source impedance (at all audible frequencies); in this case it is 10 times less.

3)As was noted you apparently have no means of adjusting headphone volume.

4)After looking at the manual for the preamp I can't tell for sure whether or not the tape outputs are driven by a buffer stage which isolates them from the main signal path. Many and perhaps most preamps do not provide a separate buffer stage for their tape outputs, although I suspect after looking at the manual that yours does. If it does not, however, I wouldn't be surprised if applying the 32 ohm impedance of the headphones to the tape outputs adversely affects the sound you hear through your speakers (just while the headphones are connected, of course; I would not expect any damage to occur, that might affect sonics when they are not connected).

A dedicated headphone amp, connected to the tape outputs, would resolve all of those issues.

Regards,
-- Al

Mechans, as Mulveling indicated my calculations are consistent with the published specs, shown near the bottom of this page. As he points out, though, it certainly seems possible that the max power handling number reflects how much power the headphones are rated to handle continuously, and that they may be able to handle considerably more on brief musical peaks. And/or perhaps that spec is very conservative, and has a lot of margin built into it.

Either way, though, the relation I described between power requirements for the phones and for the typical speakers I described remains valid, assuming the sensitivity and impedance specs are accurate.

FYI, the derivation of the 99 db figure is as follows:

The sensitivity spec for the phones is 97 db/V, which means that 1 volt in results in a 97 db SPL. Impedance is 32 ohms. 1 volt into a 32 ohm resistive load corresponds to 31.25 milliwatts (mw), based on (Vsquared/R). The 50 mw maximum power spec is approximately 2 db greater than 31.25 (based on 10 x log(50/31.25)). 97 db + 2 db = 99 db.

Regards,
-- Al

Hmmm, I just noticed that per this spec sheet the 97 db/V sensitivity spec shown at the AKG website is incorrect, big-time!

Per the spec sheet, the figure is 97 db/1mw, which seems a lot more sensible. So 99 db would be produced in response to an input of 1.585 mw, not 50 mw (10 x log(1.585/1) = 2 db).

So the speakers I described would require 40/0.001585 = 25237 times as much power to produce the same 99 db SPL at a 10 foot listening distance. And the 50 mw maximum power rating of the headphones would correspond to an SPL of 114 db.

Regards,
-- Al