There are a few of us Integra fans out there yet, I bet. Onkyo was pretty decent consumer line stuff, though the Integra series was their "high end" line up that was probably similar in performance as Adcom was back in the day.
Slightly Off Topic:
I have an Onkyo Integra 100x100 watt integrated amp that is still operating well, even after decades of use. As mentioned, more warm than transparent, it's still a good sounding piece of equipment (especially for what I paid). [Some of the] Onkyo Integra series cassette decks were benchmark cassette decks that sounded exceptional. I have a TA-2070 deck that has since failed due to overuse (DJ use), but was ruler flat in response and sounded really, really good. So, Onkyo did some products really, really well.
The '504 is a good amp. Enjoy!