On Lokeynz's advice about the chipset I tried a Thomson 536v6 and my upload speed went from 480 to 924kbps:
Uptime: 0 days, 0:01:28
DSL Type: G.992.5 annex A
Bandwidth (Up/Down) [kbps/kbps]: 924 / 18.580
Data Transferred (Sent/Received) [kB/kB]: 0,00 / 0,00
Output Power (Up/Down) [dBm]: 12,5 / 0,0
Line Attenuation (Up/Down) [dB]: 3,0 / 5,5
SN Margin (Up/Down) [dB]: 14,0 / 12,5
Vendor ID (Local/Remote): TMMB / IKNS
So I guess that means the house wiring is fine.
Is there any thing that can be done at the cabinet end (a configuration change or something) so I can continue using my Linksys modem but with a decent upload speed? Or is the only option here to use a modem with a Broadcom chipset?