Here's what I've used with the Mad Dog (2.0 through 3.2) and other T50RP variants:
O2/ODAC combo: Pretty good sounding unit. Convenient and easy to setup. Neutral and flat, but sometimes overly so (can lose a bit of dynamics). Treble can have a slight glare to it. I noticed a subtle improvement by replacing the gain opamp with an LM4562 (IIRC) and set it at 1x/2.5x gain vs 2.5x/6x gain (or something like that).
Leckerton UHA-6S Mk.II: I think this unit sounds better than the O2/ODAC combo overall. It's also smaller/more portable and about as convenient as the O2. With A/B testing, I found the Leckerton to have a slightly fuller, slightly more dynamic sound than the O2/ODAC. The treble did not have the same glare problem. Measurements are fairly close between the two units, though you can find slight differences that might explain the subjective differences I heard (either way, both measure great). However, it simply doesn't put out the same power the O2/ODAC does. With a good recording (not subject to the loudness wars), there's a good chance you'll need to max out the volume on the Leckerton if you like your music fairly loud. Most modern recordings will hurt your ears if you max out the volume on the Leckerton. FWIW, I used the 209 opamp per someone else's recommendation. I've been told it subjectively sounds better than the new stock opamp but has more issues picking up EMR from devices (the original stock opamp was the 209, I believe). In my case, the Leckerton's power was almost perfect for me with the Mad Dog 3.2. For those that need more power, the O2/ODAC is a good option as well.
Auzentech Bravura (PCI-E Sound Card): If you have a desktop computer, I've found some sound cards sound quite good and are cheaper options than a dedicated DAC/amp combo. I thought the Bravura sounded more similar to the O2/ODAC, but it had about the same power output as the Leckerton. It's a surprisingly good sound card for headphones. It measures well too on reviews (as do many sound cards geared with dedicated HP amps). I think I got mine refurbished for $90, and I think it's a better deal for what it is than the O2/ODAC or Leckerton.
Vintage Receivers (this one will be long): In my case, this would be the Sansui 5000A. It is true what people say about many vintage receivers. They sound great! However, it is best to use a headphone with a flat impedance if you have a vintage receiver, as they tend to have a very high output impedance. This is not an issue for the Mad Dog, most planars, and other purely resistive (flat impedance curve) headphones. Someone else would have to explain why that is, as I have a very simple understanding of it. Of course, these receivers are big and bulky, and they almost all use RCA inputs. You'll also need a separate DAC. I personally use my Onkyo SE-200PCI sound card as my DAC, as I've always had a dedicated stereo setup with my computer.
Anyway, vintage receivers might not be the best when the sound output is measured compared to a nice, modern headphone DAC/amp. My Sansui is not ruler-flat in the FR, though it is pretty good. I'm talking a small roll off after 30Hz and 10KHz with a less than 0.1dB variation between those points (quite flat overall). Noise and distortion is quite clean above 2-3KHz or so, usually in the mid -90dB range (likely pushing the capabilities of my sound card's ADC when I measured). It's not as good below those frequencies, but this receiver was also built 40+ years ago and probably needs some restoration done to it. And when you account for that and my less-than-perfect setup (from both an output and input perspective...my sound card's external loopback measurements highlight its limitations), I'd say my Sansui measurements are pretty great.
Measurements aside, I just think the Sansui 5000A sounds great. I'm sure other vintage receivers are the same way. I wouldn't say it's the best things in terms of resolution, black background, or ruler-flat response (though it's no slouch at all), but it just has a big, powerful sound to it. There are definitely benefits to hooking planar headphones up to speaker amps. When I say "big," I don't mean bassier. I don't mean a bigger soundstage (though you can get that from some receivers). My Leckerton in comparison sounds strained in comparison, like it has to "try" harder to produce the same volume. The Sansui in comparison at no point sounds like it is having trouble driving the Mad Dog. If I understood electronics better in terms of audio schematics and how everything fits together to affect and produce the sound we hear, I'd be able to give a better description of what I'm hearing, I think.
The cool thing is that you can find some decent vintage receivers on Craigslist or eBay for $50-100 that won't require much (if any) work to get a good taste of what they can do. :)