I pondered this very same question a few years ago. I use AKG701's and Ultrasone 8's. I have a McIntosh C2300 (previously a C2200). I used both phones with the built in C2300 with great results.
I bought a Creek headphone amp, and then a Pro-Ject SE. In short and in substance the McIntosh sounded better. More separation of instruments, tighter deeper bass. The add on headphone boxes had a very dry sound in comparison.
Also, I gave up remote control ability. The C2300 has a superb headphone amp built right in and is perhaps under appreciated.
By the way the C2300 has the 1/4" phone plug vs. the 1/8" mini plug on the C2200 requiring downsize adapters (why McIntosh did that was bizarre).
I bought a Creek headphone amp, and then a Pro-Ject SE. In short and in substance the McIntosh sounded better. More separation of instruments, tighter deeper bass. The add on headphone boxes had a very dry sound in comparison.
Also, I gave up remote control ability. The C2300 has a superb headphone amp built right in and is perhaps under appreciated.
By the way the C2300 has the 1/4" phone plug vs. the 1/8" mini plug on the C2200 requiring downsize adapters (why McIntosh did that was bizarre).