How To Get More Clean Headroom Out Of A Marshall

...I take rather efficient and wonderful sounding speakers that the good chaps at warehouse guitar speakers hooked me upwardly with. I take their Invader-50s and C/12s loaded a 4x12 loaded 10-pattern in a Hiwatt cab and they are pretty efficient speakers. Much louder and less compressed than the original Chinese "fanes" that it came with (this is probably a 2000s cab)...

Possibly then but your WGS speakers only take 35oz magnets, so probably 97dB/watt (it's regrettable that WGS don't publish this data). Whereas speakers with 50oz magnets, eg based on the Celestion G12H30, would put out 100dB/watt.
In regard of the audio force per unit area level (SPL) that the cab generates, that'south equivalent to doubling the input ability.
Hence past a long shot, the most effective thing you lot tin practise to make your rig able to generate a college SPL is to fit more than efficient speakers, the ultimate existence the EV EVM12L previously mentioned, as they won't exhibit power pinch in the way the lesser speakers inevitably will.

...I have most ... somewhere about 414vdc on the 34's...

...I have a new and strong power transformer that I believe is rated effectually 200ma...I actually retrieve I have about 440 VDC on the plates of my power tubes...

What's your HT Vdc, ie at the standby switch, at idle? And what plate or cathode current are the power tubes drawing, at idle?
If the HT V is about the same as the plate voltage, and it's down around 415V, then you've got a Marshall that will probably put out nigh 35 watts clean, 55W overdriven.
What does your HT voltage drop to when the amp is pushing out full ability?

Marshall HT voltages varied wildly through the late 60s and 70s, ranging from under 400V to over 500V, see
https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45_lead_45w.pdf
https://el34world.com/charts/Schematics/files/Marshall/Marshall_ampchart.gif
Because the OT specs stayed at about 3k4 per p-p pair, HTs down around 390 become about 30W clean, 50W overdriven.
Whereas HT around 450V become the full 50W clean, 75W overdriven.
Up at 500V information technology'll be around 60W clean, maybe near 90W overdriven.
In a higher place that but gets giddy, even Mullards and Siemens tended to accept a short, cherry-red platey life, and modern EL34 can't cope.

Things settled down in the very tardily 70s, HTs in the loftier 400s, and the JCM800s were very definitely able to meet (and exceed) the 50W nominal per EL34 pair, see p5 of the pdf of the JCM800 range spec sheet https://drtube.com/schematics/marshall/jcm800-mv-ld-transmission.pdf

If you want more than ability output, you lot need to supply the power amp with a higher, stiffer HT voltage; it'southward as simple as that.
Decent power tubes may assist a tinsey bit, simply all the stuff almost preamp and phase splitter tube swaps and circuit mods won't make diddley squat deviation once the balance of your band kicks in.

If you fit a power transformer that gives you a solid >450V HT, and load your 4x12 with EVM12L, I promise you that your bandmates volition be begging you to turn down a little