Running high impedance phones (like the HD600, HD650) directly to the tube out works great.. but IMO, using a dedicated solid state amp (that's connected to the tube output) is the way to realize the full potential of just how well the tube buffer is implemented. I'd recommend as neutral & transparent a solid state amp as possible so as not to color the sound any further (than what the tube (being used) provides).
You don't need a particularly powerful ss amp downstream (something like the O2 works great for many phones.. particularly with the HE-400). I have the TD-11's tube out connected to the Lake People G109-S.. and drive all my phones off the G109. How does it sound? Terrific. The G109 is an excellent amp in it's own right (our very own project86 reviewed it at InnerFidelity).. powerful, detailed, smooth, and very transparent... and proves to be an excellent match as a 'power amp' when the TD-11 is used as a pre-amp.
The HD800 performed quite admirably when hooked up directly to the TD-11's tube output.. but the "sparks began to fly" when the G109 (or more recently, the HPA V200) was brought into the equation. What I heard was all the excellent qualities of the tube out's sound get "magnified." The essence of the sound was still the tube's voicing.. but I hear all those things in a much more engaging, technically proficient manner. The improvement in sound was not small or insignificant, either.. it's easily noticeable (and appreciable). For lack of a better description, it felt like the tube output got 'turbocharged.' With the G109 inserted (ownstream the tube out), I heard much better dynamic range, note weight, instrument separation, soundstage, and over all articulation. Taking not of the fact the G109/V200 has ridiculously low output impedance (<1 ohm), I can even get that 'tube flavor' with my custom IEMs!
As far as using the HE-4 and/or HE-500 coupled directly to the tube out, I found that experience poor to down right unlistenable. Like most (non-hybrid) tube amps, the TD-11's tube out has a very high output impedance (~40-50 ohms, I believe). The high OI causes poor damping with low impedance phones (i.e. most all the HiFiMan planars)... and this is why high impedance phones (i.e. the Senn HD580/600/650/800) are less affected (and thus, usable directly with the tube out). Using a solid state amp with the tube out eliminates this problem as amps like the Objective2, HPA V200, and G109-S all have extremely low output impedance (and adjustable gain), satisfying damping & low noise floor requirements for virtually all headphones.
If you anticipate wanting to use the tube output with a HiFiMan planar, I can't recommend the LP/Vio amps enough (and that's not just cause I have my G109 for sale.. lol). Their mid-upper tier amps sound amazing with practically any phone you throw at them (including very sensitive IEMs), but I sense a particular synergy with orthos that's immensely enjoyable.
The LP/Vio amps (as well as may Audio-GD offerings) are capable of delivering oodles of current into low headphone impedances.. which is precisely what planars crave.. so combine that with the TD-11's wonderful tube out capabilities.. and you would understand why this combo is very pleasing ..and very versatile.
Edited by FlySweep - 3/31/13 at 10:40pm