In case you're wondering, I'm in New York...
If I'm not mistaken, Shanwick uses a number of frequencies during the day...8879, 8831, 8864 are but a few. Higher freqs are also used but in the evening you'll hear Gander and Shanwick on 5616, 5649, 8891, and then later at night they may be in the 2's and 3's.
New York tends to use 6577 a lot during the day but so far haven't heard them on your set up yet. They could be on 8825 and/or 8846 or higher freqs (11's and 13's)
Just heard Shanwick on about 6546 or 6547 (tuned to 6546.86)...didn't know about that freq. The aircraft he was talking to was told to use 8879 as a backup...
Ron