I'm sure I read somewhere that Gilmour runs his Hiwatts through Marshall cabs - though I might be mistaken.
I generally switch to the 4x12 Greenback cab and to the 57 mic (either on or off axis) and i also raise the channel volume to about 70/75.
This gives the amp a much fuller and punchier feel
Also, I don't think you'll get Gilmour lead tones just using the amp alone.....Gilmour uses various Compression and Drive pedals to get his tones.
Have you looked at www.gilmourish.com - there are loads of great tips for getting his sound if you search that site.