Question about Class A

@jay73  I agree that the SET 120 would be the better purchase as you would not need the additional power. Are you talking about the integrated version or the amplifier? AVA makes both. 

What are your sources?  If digital you would have to replace the DAC in the SV-500. At your budget with purchase of the 120 integrated you could purchase a tube DAC if wanting some tube sound in your system.    

I must say that I once owned a Vincent hybrid amplifier, the SV-226. It was class AB. I sold it and consider that move one of my audio mistakes. It had more power than the SV-500 at 100wpc and was line level only. Fed it with an outboard DAC and phonostage for sources. I would encourage you not to be in a hurry in your decision making. Continue to explore.

I am still at a loss to recall a class A 20+wpc amplifier that retails for ~3K other than the Sudgen. Not to say they don"t exist.    

The OP is looking for a Class A integrated amplifier.  Budget extends to 3K new/used.  The Sudgen A21SE used is the only one that comes to mind. Others?

At 3X the price the Sugden A21SE had better prove an improvement over the Vincent SV-500.  Actually any of the A21 series should be.

I would not advise the purchase of an amplifier with intentions of using your Vincent as a pre unless you are planing on replacing the Vincent with a higher end preamp in the near future.

Keep your eyes open for a used Sugden A21 series integrated. Should you find one you could purchase it, compare to your Vincent and keep the one that fits best into your system. Sell the other. 

As an aside to your original quarry, have you considered the purchase of an external DAC to replace the one internal to the Vincent?  Improvement in your source may prove to be a better expenditure.

Congrats on your purchase! Please let us know your thoughts after comparison to your Vincent. I am thinking you are going to be very happy.

I had forgotten your were using an external DAC when making my last post. 

Unless I missed it, nowhere herein did you state the other components of your system.  Remember you are listening to a system, not a single component.

I would not involve a switch box. Listen to the Sugden for a while. Then replace with the Vincent. Repeat with extended listening sessions. The experience will lead you to understand what sonic attributes are most important to you. 

Another question, what is size of your room?

@jay73 Sorry I missed the requested info.

I would listen to the Sudgen for a while. Let it break in. Let yourself adjust to it. After some 100 hours swap back the Vincent. Then revisit your thoughts.