I am not quite sure, since I don't have HD-DAC1 (yet). But I assume, it would be quite an improvement as it has variable gain setting, matching different impedances.
PM6005 probably adds only a series resistor between the amplifier and the headphone jack, to protect it from short circuits and overloads. I am not sure for your amp, but have seen other Marantz amps with a resistor as high as 210 ohm. Which can poorly control HD650, damping factor being very low.
First my apologies DjBobby, I don't mean to pound on you personally at all, it just happens that you wrote a couple of things over the last couple of weeks I thought worth commenting on. The issue of damping factor re. headphones being one of them.
The damping factor is much less of an issue with headphones than it is with speakers. Speaker power amplifiers are much more difficult to construct with low output impedance, and as such their output impedance is relatively large compared to the typical 8 Ohm speaker impedance. Couple this with the relatively large mechanical mass involved of the speaker diaphragm & voice coil, and damping factor matters indeed.
With headphones the diaphragm is relatively light weight, and the air loading itself imparts quite a bit of damping. Furthermore, headphone impedance is usually higher and it is easy to construct a headphone amp with low impedance.
However, when something like a typical 120 Ohm resistor is incorporated in the headphone amp's output, what changes the characteristics of the headphone's sound is not so much a problem of damping factor where the amp is trying to keep control of a diaphragm that has a "resonant" mind of its own, but rather the fact that the headphone's impedance is far from flat over the bass register. This means that the output impedance of the amp has a big impact on how much relative energy is dissipated in the various frequency bands below, say, 500Hz.
This why some headphones are designed to have their best tonal balance when driven from an industry standard 120Ohm output impedance (Beyerdynamic notably). Now you
may in fact prefer the sound when driven by a zero-ish output impedance (a very personal matter of taste), but this is nothing to do with damping factor and everything with the (very much non-flat) impedance characteristics of the headphone.
On top of that, I have a much more personal theory that damping factor matters even less with headphones because of the geometry of the magnetic gap & voice coil. Most headphones linearize their electromagnetic response by having a long-throw voice coil that overhangs the magnetic gap substantially. This means that of the 300 Ohm impedance of the HD650 e.g., only a 100 Ohm or so will ever be "active" and generate back EMF when vibrating. The other 200 Ohm is hanging there as dead weight just adding to the overall circuit impedance which matters for calculating the damping factor. You could almost consider it a 100 Ohm headphone with built-in 200 Ohm series resistance (the latter off course also having some minor inductance as suspended in air)