Morning, Bob - I emailed K5OOR Virgil with your question but
in the mean time I looked at the schematics. The incoming signal
is fed through an attenuator. This attenuator (r2, r3 & r4) can be
set up for specific rigs. Here is the matrix:

DB R3 R2 R4 IN OUT RIG
3 300 18 300 2.0 1.0 K2
4 220 24 220 2.5 1.0 FT-817
5 130 43 130 3.0 1.0 Other

As you can see, the amp really needs only 1 watt of drive. It
would be very simple to make a pad to attenuate your 5w
down to 1w. A 6db pad would take 4w down to 1w but
5w would be 1.25 watts, a little much so I'd go with a 7db
pad - according to my math that would be just right.

Update - Virgil just sent me the specs on a 7db pad:

DB R3 R2 R4
7 130.7 44.8 130.7

hope that helps,

73's de Ken KG0WX