preamp and amp to integrated

Well, The preamp section of Arcam is active so you can try both ways to achieve the sound you've got used to or better.
50W vs. 60W per channel should not deliver you much difference.

The Arcam is better than what you have now. It should have plenty of power, but keep in mind that there's no standard manufacturers use when measuring power, so even both amps were rated for the same amount, in reality they would be different. How much of a difference needs to be taken on a case by case basis, but from having experience with both brands, I would guess that they would be fairly close.

You might be able to use the Arcam to power your main speakers and also use the pre-out from the Arcam into the Adcom amp and let the Adcom power the in-walls. Playing more than 2 pair of speakers from one amp is typically not advisable due to the lowering of the impedance as you add speaker pairs. Of course, if you have some type of impedance matching speaker control box in the system, this does not apply.

Most amps (including integrated amps and receivers) are typically spec'd to power (up to) two pair of 8 ohm speakers. Two pair of 8 ohm speakers equals 4 ohms. Adding yet another pair to the same amplifier outputs would lower the impedance ever further and that is typically not advisable. Splitting your multiple pairs of speakers between 2 or more amps, such as using both the Arcam and the Adcom, would overcome this potential issue.

" Two pair of 8 ohm speakers equals 4 ohms. Adding yet another pair to the same amplifier outputs would lower the impedance ever further and that is typically not advisable."

That's not always correct. The speakers can be wired in series, in which case the exact opposite is true. Not only that, not all "8 ohm" speakers are the same. Some can be harder to drive than others.