Always been sceptical about Silbaby as a barrow, but there is an argument in its favour;
Silbury itself...
"The tunnel showed that the monument began as a mound less than 40 metres in diameter, carefully composed of gravel, turves, soil and chalk, in part at least revetted by stakes. This overlay an old land surface above a subsoil of clay with flints on what had been originally a spur of middle chalk projecting into the Kennet valley" taken from Whittle - Sacred Mounds, Holy Rings..

that the primary barrow under Silbury Hill was on a "spur of middle chalk projecting into the Kennet Valley" could point as to why the maybe Silbaby barrow could also have been on a spur from Waden Hill but it is awkwardly placed at the foot of the hill, on low ground, presumably flooded at times by the Kennet. And why did the Romans "trash" it, they were a superstitious lot, burying their dead in the old bronze age barrows...