ARC Ref. 3 - New Tubes - Need help

Hi all,
I want to change the tubes of my ARC Ref. 3 Pre amp.
Just wandering what is the correct match of the tubes for the two channels (right and left).

The board where the tubes are connected is placed vertically and the sockets are named V1, V2, V3 and V4.

(view from Back - TOP)
V1 V3

V2 V4
(bottom)

Are the pairs V1+V3 and V2+V4 or V1+V2 and V3+V4?