yes a separate preamp would make a very good improvement with the right preamp
i would look to spend 2 to 3k and look at a tube preamp a used cj vtl or vac would amaze you
think of it this way your 800 receiver contains 7.2 preamplifiers 7.2 channels of dacs a video switcher a digital processor this means the piece cost 200-300 to manufacturer so your parts quality can not be very high
and then all of this circuitry will affect your sound quality
so yes you will be amazed especially with a good tube preamp
Dave and Troy
audio Intellect NJ
40 year audio veterans