Kris

I'm not familar first hand with the Rocktron Velocity 300, but I am very familiar with the Spider Valve and several other tube amps from Line 6, Marshall and Mesa Boogie as well as a number of solid state amps.

From what you have said, your experience with the HD500 and the Spider Valve power amp section pretty much echoes what I would expect from a normal guitar amp's power amp section too, so with minor variations between 'normal' guitar amps I'd say that what you hear from your Spider Valve is pretty much correct. The Spider Valve combo amps use a typical guitar speaker (Celestion Vintage 30) which has a pretty strong character of its own rather than being a flat response speaker like perhaps a Celestion G12K100 might be.

I have downloaded the manual for the Rocktron Velocity 300 power amp and taken a look through it.

This amp appears to be 'pre-amp ready' which to me means LINE level ready, and that being the case, with your HD500's 1/4" outputs being connected to the Velocity's inputs with the HD500 output mode set to Stack Pwr Amp and the AMP/LINE switch being set to LINE you should be in the right ball-park for the Rocktron to act like any other guitar power amp which is powering 4x12 guitar cabs - admittedly you might want to play a little bit with Cab modelling on or off to see which gets you your preferred sound, but pretty much with a normal amp you'd be nearly there bar minor tweaks here and there.

OK - the Rocktron Velocity 300 isn't quite a standard or 'normal' power amp from what I can tell.   It has Definition Controls to help your sound not to get lost in the mush of a mix.  For what Rocktron term 'Full definition' those controls need to be turned fully clockwise (100%), and for the Definition circuitry to be bypassed they need to be fully counter-clockwise (0%), so I would suggest that you need to have the Definition Controls for both channels turned all the way off (0%) for a more natural and expected sound.

You also have a couple of 'Reactance' controls which when turned right down (0%) do less to affect the output modelling than when they are turned up full (100%).

Finally you have a pair of Volume controls which will pretty much work as expected I guess.

So to recap, here's what I think should be the correct amp settings for the Rocktron (bearing in mind I don't have any experience with the Velocity 300)

Volume = anywhere between 0% and 100% - I'd probably start at somewhere around 25% and increase as necessary from there

Reactance = 0%

Definition = 0%

Then the HD500:

Outputs = 1/4" both (assuming you're not running the Rocktron in bridged output mode/mono or if you are running in mono use one 1/4" output only the same as you do with your SV)

AMP/LINE switch = LINE

Output Mode = Stack Pwr Amp

Amp Models = Probably PRE but maybe FULL for some presets depending on what you need to achieve

Speaker Cab Emulation = I'd try ON first - play with the actual cab model, AIR settings and Mic type, and then try with Cab Modelling OFF - basically whatever settings work with the SV should also work with the Rocktron and sound not a million miles away from each other.

Master Volume = 100% or near 100%

Other than that with the exception maybe of speaker phase issues, which could be caused by speaker cab modifications or cables that have been mis-wired (unlikely) I can't see why the Rocktron shouldn't give you excellent responses from your HD500.

Nick

Do you have a CD Player or MP3 player with line outs?  I would hook that up to the Velocity 300 and see if the same issue occurs.  If it does, obviously something is wrong with your power amp or cabinet.  If it does not, then it's something about your signal chain.

My chain would be set up as follows using your setup:

Guitar > instrument input on Pod HD500

Amp/effects simulation enabled and panned center (unless you're using the power amp in stereo)

Possibly use the cabinet simulation-possibly not

Set Pod HD500 to Stack Power Amp or whatever output you prefer

Set Pod HD to "Line" with the little toggle switch (the HD500 is outputting line level signal and your power amp is expecting line level as well)

HD500 1/4" output to Velocity 300 input

Velocity 300 output to cabinet at proper Ohm settings.

Turn the volume of the Velocity 300 all the way up.

Turn the master volume of the HD500 all the way down

Turn your guitar volume all the way up

Turn up the Volume pedal on the HD500

Slowly raise the HD500 Master Volume.

Does it sound ok when it's quiet?  Does it get bad when it's all the way up?  Make sure the HD500 isn't clipping the inputs on the Velocity 300.  if it is crap when it's loud, then turn down the HD500 volume until it cleans up and use the volume on the power amp to control your stage volume.