My experience is that the Arcam would definitely be the best-sounding amplifier for use with most speakers. Its amplifiers and power supply are well-designed and can handle the difficult loads that many speakers present to an amplifier.

With the Marantz you will find that it will not sound that good with the majority of speakers, because it is not really designed to drive low impedances, and most good speakers drop to 5 ohms or less at many frequencies and also present a reactive load at many frequencies.

I also suggest that you get a Musical Fidelity V-LPS phono preamp; it is only $200, and IMO is the best you can get for under $500. It is well worth its price.

It will really make a big difference to your sound quality, compared to the ones that are included in most integrated amplifiers.

If you have not selected some speakers yet, i suggest that you check out the PSB Image T6 speakers. They are marvelous speakers for only $1300 per pair. Both The Absolute Sound and Stereophile selected them as best in class and included them in Recommended Systems articles.

P.S.-due to selling one of my homes, I have a like-new Musical Fidelity M3i amplifier that I will sell for $950. IMO it is considerably better than any of the above.

Period.  It's the only one that will do space and ambiance nearly the way a single ended triode or pentode will, and it will drive almost anything well and the phono stage is comparable to most under 1000.00 standalone units.

And it's a better amp than comm's MF. 

Sorry man, it just is! :)

It would be interesting to try them head-to-head.

I think they are both very good amplifiers, in any case.

The MF M3i does work great with all of the speakers I have used it with, and it does put out close to 100 watts per channel according to the tests done by an independent lab.

I very much doubt that the phono stage of the Arcam amp is anywhere near as good as the V-LPS, which is outstanding.