The Marantz SR6004 isn't a wimpy department store A/V receiver. I was referring to a typical $400-500 A/V receiver. The UPA-500 would be an upgrade to that.
I only used the SR6004 as a preamp because it was the least expensive receiver/preamp I had at my disposal when testing the UPA-500.
To get a substantial upgrade in power from the SR6004 in the Emotiva line, one would have to look at the Emotiva XPA series of amps.