seriously, do not even think of plugging that output into a line input, if you value your line input... also, it will damage your output tube if you were to connect only the line out and no speaker load. If you built your ax84 you should know that no tube amp can function without a load...
I'd say, the proper thing to do would be to pad that output down with a resistive divider, somewhere along the lines of a 5 to 10k load. then send that to a single ended to balanced transformer so you can send it to a balanced input, thus reducing the noise pickup.
To answer mathematically to your qeustion, a 5W signal in an 8 ohm load is about 6.3 volts peak, or roughly 4.5 volts RMS. You would need a voltage division ratio of about 1:4. So four 1K ressitors in series and you take the output on the last one that's connected to ground.
5 Watts into 8 Ohms corresponds to somewhere around +19dBu (+15VU) so is a touch 'hot' for a line input particularly if you want to play loud.
You should therefore 'pad' it down with an attenuator so the 4:1 as proposed above is probably about OK.
You MUST however have a load on the the output transformer either by keeping the speaker connected, or by fitting a resistor of 8 Ohms (10 Ohms would be fine) rated at say 10 Watts and mounted on a heatsink (a few square inches of metal is fine).