I have compared the Arcam and the Pioneer. All I have to do for a/b comparison is to mute one or the other. The Pioneer leaves the Arcam in the dust. The only difference is the Arcam is getting audio via HDMI, the Musical Fidelity/Pioneer by way of RCA interconnects.
In my opinion, you have two things against you with using the Arcam. The first thing is that any digital audio transferred over HDMI will sound worse than digital audio sent over digital COAX or even analog audio cables. The data bandwidth and clock timing required by HDMI interface plays havoc with the sound quality when the receiver attempts to re-assemble the digital data (which could have bits missing due to data bandwidth sharing with video) and the digital clock timing, which is already compromised. It is always better to use digital COAX to transfer audio. The only exception is the high res bluray audio formats (DTS-MA and Dolby TrueHD). 2-channel PCM and old school dolby digital /DTS will always sound better over COAX.
The second thing is that ARCAM has its own unique sound. While it is very well regarded, in my opinion, it has a very weird sound. It is very clean and high definition, but to me it just doesn't sound realistic. ARCAM likes to use what they call Class G amplifier. It is a weird combination of a Class A amp/power supply circuit with what looks like a Class D type secondary power supply that is switched in/out as needed. I think the result is that it gives more of a Class D type signature, which lacks air and just doesn't "sing" in my opinion.
I think these two items are why you feel that the Pioneer receiver sounds so much better (Pioneer is standard Class AB architecture).