built-in receiver phono preamps, how good are they

You have to listen. Some built-in phono stages were very good and others were not.

I found the built-in phono section of a Yamaha RX-V1300 to be on par with a Parasound zPhono. That's not saying much. It was quiet and I could play records, but the sound was flat and painfully two dimensional. Before you start adding peripherals like a phono stage, you should take a close look at your whole system and think about what should be upgraded. Personally, I found a much bigger improvement when I switched to a separate amp and preamp than adding a decent phono stage to the Yamaha receiver.

thanks for the replies narrod and mingles. i'm already using separates, outlaw preamp and an adcom gfa-7000 amp. the denon phono end sounds decent but kinda flat compared to my digital rig. low gain maybe? there's nowhere to audition gear around here, so i'm hesitant to buy something and hope it sounds good when fedex delivers it.

Depends on the receiver

I have a 70's vintage Yamaha CR-720 on my second system + the phono is outstanding!

THe phono section on a Tandberg TR-2080 was also quite good.

The phono on my spare NAD 7020 is just OK.