Check the input cabling, as this would be the most common cause for problems.
Also, the iPhone output level is not very strong, so you will not get full output from the T2 with an iPhone. But make sure the iPhone level is set to max (including any setting in the program you use). If possible try another source with higher output.
Hi Toby, the topside of the board is filled with SGND plane, so no need to wire to R6 or R7. You can wire to potmeter pin1 or 4.
We use 0805 actually, but yes you can use 0402.