Advice on tyres please

[wynnj]

Having driven S60s and a V70 for 16 years now I became a new owner to a 2007 XC90 D5 (133K miles) a few months ago, and on the whole am pleased with it. I've flushed the auto box with the correct mobil ATF which improved the gear change though it still thumps on a light throttle upchanging from 3rd to 4th (annoyingly at ~32mph) when hot but I'm living with it at present, and apart from sorting a few electrical issues, have changed the cam and auxilliaries belts and all their pulleys using volvo parts bought from FRF in Swansea.

I'm soon going to need at least two new tyres on the XC and probably retracking.

I run the S60 and V70 on Goodyear asymetrics and am generally very happy with them.
The xc90 currently has 235/60 R18 103s fitted:- AVON ZX7s on the front which need to be changed (outside edges wearing) and Bridgestone Dueler HPSport on the rear both of which are wearing true having been fitted about a year ago and have lots of tread left. (They're marked as MO i.e. for Mercedes!!)

Most of the online tyre sites bring up 235/60r18 107 s as the preferred tyre for this car but I've no way of knowing what it was fitted with originally.

My quandry is should I just fit two more Bridgestone 235/60r18 103s as my cheapest safe option, or should I change all four and fit 107 rated All Season tyres all round, and if the latter, apart from the hit on my bank balance, are there any downsides to running AS tyres all year?

I can't say I'm too impressed with the XC's general handling and stabilty, but realise that that may be just a comparison to the S60, or the fact that it's currently got mixed tyres and a probable tracking error.

All help gratefully received.
Thanks in advance