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You've got TWO Arcam P38 105w/ch to 8 ohms amps. I can't imagine that they would not do a find job of driving the Helicon 800's, especially since they are Arcam amps.
Now, more power is always a good thing, and the Helicon can handle amps up to 400w according to the specs -
Recommended amplifier power [W]: 50 - 400
But also consider this -
Sensitivity (2,83 V/1 m) [dB]: 89.5
Maximum SPL [dB]: 113
Those are reasonably efficient speakers.
I think the amps you have will do a fine job, but it gets down to the size of your wallet.
I assume since you have two amps, you are bi-amping. The 100w amps should be far more than enough for the mids and highs. So, if you insist on upgrading, I think you would only need to upgrade one of the amps. The amp that is on the low frequency section.
Keep in mind that in the standard orchestral power distribution, the power in the low range, below 500hz, is 5 to 10 times what it is at higher frequencies. So, the mids and highs require far less power than the bass range.
Though, what amp would be a good upgrade, I'm not sure.
Personally, I say try it with what you have and see how it works. Then if you feel you need more, upgrade the low frequency amps (assuming you are bi-amping), and give that a test. If you are still not satisfied, them perhaps consider upgrading the high frequency amps (again, assuming you are bi-amping).
Start from where you are at, and keep going until you get to where you need to be.
My advice is 'start from where you are at', meaning use what you have now and see how it goes.
If the Arcam P38s have the ability to be bridged into one large mono amp instead of two stereo amps, then that would give you easily 200w.
And you would only need ONE more P38. You have one for the mid/high frequencies; that's more than enough. Take your existing one and bridge it on the left channel, and take the new one and bridge it on the right channel.
Now, you've effectively got 300w per channel; 200w on the bass, and 100w on the mid/high.
But again, it hinges on the Arcam amps being able to be configured into a bridged or mono mode.
That would probably be your cheapest upgrade path.
But again, try what you have, and see how you like it. Then we can work from there.
A quick check of the P38 owner's manual indicates that there is a 'Mono Link' to help facilitate bi-amping, but no indication that the amps can be bridged into a single mono amp.
I was using an Arcam P7 to bi-amp the Helicon 400 mk2 and really liked the results. I agree with Steve that you should stick with what you have to start with and see what the results are like, but as efficient speakers you should get a good result with 200/300 watts.
Obviously moving to Bryston or to a lesser extent an Arcam P777 should improve results but you may find its just not worth the outlay when you have a really good basis already.
P.S. Very jealous the Helicons are wonderful speakers and not a day goes by that I dont miss the ribbon tweeters.