I am using a NAD 7140 which I think has the same preamp of your 3155. Similar to you, I purchased it for the phono as I needed a phono section that could be driven by a low output MC cartridge (I was given a denon 790 turntable with a low output denon cartridge). Like you, I feel it has a great headphone section using my Grados 225 headphones. However, I felt my NAD did not drive borrowed sets of beyers 990 or akgs 701 too well. Later, using the 225s, I was able to compare the NAD to a Grado RA1 headphone amp and could not discern much, if any, difference in sound quality. ATM, I am pleased with the sound quality of the NAD with my 225s.