You mean you want to install AVS such as Slagleformers as volume control inside another preamp?So if one was to use the AVC in another preamp, it would be just the AVC, not the Elma former, to buy?
Yes, there are some brands of preamp using autoformers as volume control internally.
However the Slagleformers are of auch high quality that IMO they are excellent passive preamps by themselves.