Detecting is a popular hobby for many retired folk in the UK, i can certainly understand an old lad in his 60's-70's not wanting to swing a big heavy machine on ploughed ground for 8hrs, saying that, the 13" coil is a different story, badly effects the balance of the Deus.
I think every beefy coil is going to affect the balance, somewhat. I've looked at counter-balancing my rig for my big coil, but the truth is, it's easier to swing unbalanced, simply because of arm fatigue from weight. Besides that, the physics of a device in motion remove a lot of the "unbalanced" feel. Just get it swinging and it kind of coasts along doing it's thing. I've found I now take wider/longer swings for more coverage between stopping and starting the swing. Helps some.
Trying to balance it out, though, just exhausted me. Total weight is pretty important... so is using the arm cuff strap. On my first rig, I didn't use the cuff strap (it was pretty light, and not too bad to swing). When I upgraded to a mid-range device, the balance was worse, but I still didn't use the cuff (just got a stronger grip). Only when I upgraded the coil, did I really discover how amazing the cuff strap is for weight distribution during a swing. if that cuff is tight on your arm (not so tight that it cuts circulation, just firm against it), it'll take a WHOLE lot of weight off the arm.) Took only a few outings to get used to the "feel" of the cuff, but WELL worth it in arm and hand fatigue.
The physics involved, are a bit tricky, as it seems counter-intuitive that the cuff strap would do anything since the leverage point of your hand and a heavy nose pulls down on the nose (which would push the arm cuff UP on the bottom of your arm, as you grip the device), but it most certainly does. The strap stabilizes the whole set of equipment, so instead of using your grip to stabilize the device during the swing, you can actually open your fingers and just let the leverage do the work (very little grip required with the strap on, and the device in motion). By keeping the coil level (no slop in the cuff area), This allows you to truly just use the grip as a leverage point, instead of having to stabilize the actual machine WITH your grip.
Bottom line here, is that for many detectorists (maybe not you, but others reading), you can compensate a LOT for the unbalanced nature of your equipment by properly using the arm cuff strap. It may feel weird, like you have to crawl your arm out of it to dig, but you'll get the hang of pointing your equipment to the side, and simply sliding your arm in and out (it's a very fluid natural motion, that way, btw).
This video is not originally designed around sliding my arm in and out of the cuff, but you can clearly see it's not an issue:
https://www.youtube.com/watch?v=072vXxug-Rk
Cheers,
Skippy