If Musicnik means the PSR550 from 2001 (and not the much newer PSRs550 released in 2008), then he's correct; there were old samples on the PSR550/630/640/730/740 up to PSR2100/PSR9000 that were *very* heavy and 80's sounding. Nowhere near as crisp as the sampled drums introduced starting in Tyros 1; ironically the old 80's style samples sounded *better* than the new T1/PSR3000 drums (looking back, that was really the weak point on the T1/PSR3000, which they immediately started addressing with the new kits in the T2,T4 and now the Genos)
The Dance kit back then wasn't the same as the ones we get now, same goes for standard kit 1 & 2. They were very punchy and analog sounding, despite being (presumably) less memory intensive.
Yamaha *did* give us 1 kit of that old style in YEM, and that's the psr340 pack, if you want to check out what the old punchier (but not as clear) drums sounded like... they were particulaly fuzzy for the hi hats, and even the toms. It was just the kick and snare that had that really thick retro sound.
New new version of Standard Kit 1 is much, much cleaner, sampled at a higher res... but ultimately sounds a little lifeless (which they've since massively improved with Power Kit 1/2, Rock/Real/ and now the Revo kits)
Unfortunately you need T5/PSRs970 or newer to use YEM to install the 1 pre Tyros drum kit.
If he's talking about the s550 though, then that was a post Tyros keyboard, and doesn't contain any of the pre-tyros drums (that I recall).
You can tell them apart because the PSR550 used floppy disks, was painted silver champagne across most of the front face, the screen appeard green when off, and used a 12v power supply (Pa6d I think)
The PSRs550 was USB A port for data transfer, all black (although there was an all silver version too), with a blue & black lcd (looks dark blue when off), using a 16v power supply
Mark