I recently had a problem with line noise, telstra fixed the line. Amnet changed some settings on my profile, it seems since then I've been experiencing slow speeds lately from both international and local websites.
I cannot seem to download faster than 50KB/s, browsing has also been slower than usual with lots of timeouts. Did a few speedtests but the speeds reported are way lower than what i'm connected at.

Modem is still connecting at speeds I had previously before Telstra fixed my phone line. From the top of my head the setting Amnet changed had something to do with interleaving, latency is much better now while gaming but general internet speeds have become .. well crap. Is it possible the change in 'interleaving' setting is slowing my net down?

Doubtful. Generally only affects pings, it may be affecting your synch somehow.

What sync speed have you got?