I use a 557 every day and still think it is a great power amp - I used to have a 500>518>566-24 until I replaced them with a G08, into M A500 speakers - lovely sound. The 559 is physically bigger and heavier, with more power and does sound better than the 557. An alternative to a 559 could be to buy two 557s and use them in bridge mode to bi-amp your speakers!

Meridian G08 CD Player, McIntosh C2300 valve pre-amp, 2 x McIntosh MC601 monoblocks, B&W 802Ds with IsoAcoustics GAIA 1s, AudioQuest Aspen speaker cables, Chord and Audioquest inter-connects, Shunyata power cables, Furman power conditioner, Sennheiser HD650 with Schiit Audio Asgard 3 Headamp