I had the 880 on two different occasionI I owned them and had them side by side with the T1and the T1 always was more transparent and had better treble and bass and much more refinement than the 880. IMO its Beyer best offering. The HE500 is an excellent phone and different than the T1 but its subjective what would be better. To many people they prefer the T1 transparency and pinpoint focus while others prefer the HE500. Both are top tier headphones IMO . The 880 may cost less but its not as refined as the T1. I owned the T1 for 2yrs before I sold it and it was a great headphone but I preferred my HD800 slightly. YOMV
I have to agree with Frank on this one. Also, just to note: The T1 amp requirements may be more demanding than the 500s. I find my B22 does in fact bring out the best of the T1. The bass on the T1 is more accurate when compared to the 500s - not as deep in the sub bass area but more accuracy as across the spectrum. I find the treble on the 500s to be much smoother but the energy is about the same. Again the T1 is not as forgiving as the 500s are thus make it more transparent - the 500s are a warmer headphone by far.
I like the 880s as well but the T1 IMO is a step above.