I have never heard a rane mixer myself, but i'm sure they are great (judging by their reputation). Nevertheless, i'd place a bet that a good dedicated phono-pre will blow it out of the water.
Just an example: 500$ mixer vs. 500$ phono-pre. One company has to design and produce a mixer, including ins and outs, faders, blablabla, AND a phono-pre, budget is 500$. The other company has to design and produce... ONLY a phono-pre, budget is 500$. Which phono-pre will sound better (let's say both companies strive for a humble product with great quality)?
On the other hand... since you're entering the "audiophile" world with these products, there's bound to be endless products out there that'll sell for thousands and don't justify their price. So in regard of sampling, i wouldn't go crazy and buy a phono-pre for a few thousand bucks.
The best bang for buck is probably something like Clear Audio or Cambridge Audio. I've read countless reviews on these products... in the end i got the 640p from cambridge audio, as it seems to perform almost as good as phono-pre's that cost 1 grand or more.
What is also important is the turntable, tonearm, cartridge, etc... which should all be considered as part of the chain. For example: If you're using a 10 year old super cheap cartridge that's been used and abused, you probably should buy a new cartridge first before buying a phono-preamp.
And to answer the OPs question...
Yes, i've made the switch from a Yamaha (ax397) hi-fi amp (which has a seperated phono-pre (withing the case of the amp) that get's praised as "great quality" in reviews) to a dedicated phono-pre (cambridge audio 640p). The difference in quality is immense! The definition of details, the definition of bass, the stereo image, etc... is worlds apart from what i got out of the yamaha amp. Simply no comparison. I have in the meanwhile replaced my yamaha amp with an old Rexox amp (vintage amps are supposed to have really nice phono-pres)... still no comparison to the dedicated phono-pre (and the revox amp is serviced/old parts replaced and sounds like new).
I use an SL1210MKII with a shure me97xe cartridge by the way, properly set-up and dialed in.
PS: I don't have the yamaha amp anymore, but if someone is interested i'll make a recording for comparison between the revox amp and the dedicated phono-pre.
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