Is there a way to look up DSL stats (e.g. SNR margin) on time capsule?

Not if the airport is doing pppoe.

Airports are not flexible enough to add a second route.. so it is simply never going to work..

However you can sometimes still tap into the modem stats via a local network.

Tell me what modem it is? Does it have more than one ethernet port? Wireless?

Using a wireless modem router in bridge actually makes sense because having wireless allows you to easily find info from the modem. Any computer in the network can just swap over to the modem's wireless ssid and pick up the stats.

No logs at all on a Time Capsule, so if you want to check the SNR of a wireless connection through the Time Capsule, you will need a utility like WiFi Scanner or WiFi Explorer. More info below.

Mac App Store - WiFi Scanner - Apple

Mac App Store - WiFi Explorer - Apple

As far as modem logs, that is up to your modem. If you can get to the modem logs, you might find a few hints there. Check with support for your your modem.

Just plug a computer in directly to the ethernet port of the N1.. you can then get the stats.. you can also turn on wireless .. if you wish.

I would personally use the wireless.. set it to 20mhz only and channel 1.. and it will live very happily with the TC doing wireless.. as there is 2 other channels available at 2.4ghz.