How to measure amplifier circuit loop gain
Loop gain is the pk to pk signal at the base of Q9 (wrt ground) divided by the pk to pk signal between the bases of Q9 and Q10. (Feedback loop intact). You'll run into problems measuring loop gain with the loop opened because dc offsets complicate the issue, saturating the output.

The pk to pk signal between the bases of Q9 &amp; Q10 is too small to measure at low frequency in a practical way unless its amplified by, for instance, by an instrumentation amp.

To measure the difference voltage between the input stage transistor bases is difficult with an earthed scope. I floated my scope to do it (I disconnected the Earth in the mains plug but this is not recommended and only if you know what you are doing). Again you could amplify the input difference signal with, say, an AD623 instrumentation amp and then you'll have a an ac signal referenced to 0V.

For stability the loop gain should be about 30dB (no more than 40dB) at 20kHz. 

EDIT

The whole loop is being taken into account with both circuits and so there should be 180 loop lag at low frequency because of the amplifier's inverting input. This is shown as 180 lead on the green graph which is the equivalent. On this graph, how short of 0 degrees (actually -360 in the circuit) the lag is at 0dB is the phase margin which is -90 degrees.

The red graph is showing 0 degrees lag around the complete loop at low frequency. I can't understand why it isn't showing -180 (or +180 as does the green graph), but if we take the low frequency loop lag as 0 degrees then phase margin is how short of -180 (-180 on the graph, -360 in the circuit) the loop lag is at 0dB. This again is -90 degrees. The two graphs appear to be showing the same thing.

There must be a reason why the V0/Vx graph is not showing -180 lag at low frequency.

Because the phase margin is negative it implies that the amp is potentially unstable. I'd suggest adding a compensation capacitor to get the loop phase lag down below 360 degrees at 0dB loop gain. I'd try 100pF between the base and collector of the common emitter output stage driver, Q8. 

EDIT

I think what's happening in the red graph is that the signal source is adding 180 degrees of lag into the loop which, when added to the 180 lag of the inverting input, causes 360 lag (0 degrees lag) around the loop at low frequency. To get a true picture of the loop phase of the amp you need to subtract 180 from the red graph's phase plot. This is effectively what I have done above before determining phase margin from the red graph.

You may wonder why the red amp isn't oscillating when there is 360 degrees lag around the loop. Well, the Barkhausen Criteria state that, for oscillation to occur, there must be 360 lag around the loop and the loop gain must be unity. Well, in the case of the red graph, there is 360 lag around the loop but the gain is much greater than 1. Hence no oscillation. By the time the loop lag gets up to 720 degrees lag the loop gain is much less than 1.

EDIT

To summarise:

Subtract 360 degrees off of the phase scale of the green graph.

Subtract 180 degrees off of the phase scale of the red graph.

Sorry about all of the edits and changes, quite complicated to get right.