Welcome to the forum!
An HDMI connection from the blu-ray player to a receiver would give you access to the newer lossless audio codecs. I'm glad that I have that, but I don't know how much of a difference you would hear compared to Dolby Digital/DTS that you already have.
One big change in receivers since you bought yours is auto calibration. A lot of receivers come with a microphone that will help calibrate your speaker system. I had a Yamaha from 1997 before buying a much less expensive Pioneer receiver in 2005 - and I loved the Pioneer because it's MCACC auto calibration system worked wonders on my NHT speakers in my room. For me, it was a huge benefit.
Some people think that Audyssey is a better calibration system than either YPAO (Yamaha's system) or MCACC (Pioneer's system). Audyssey is available on Denon and Onkyo (and maybe others - I haven't shopped in a couple of years). I like MCACC, but it's the only one that I've ever used.