Who to trust when buying vintage ss ?

I would not start out in “vintage”. In audio. That’s just a misplaced synonym for “old” and high risk of kaChing of pending repairs , if even possible . ( hint: parts can be scarce ) 

without prejudice to the former, most budget current offering options will generally easily best the “vintage” options in audio performance, The latter is primarily just a playground for nostalgia fans willing to compromise on the warts of old gear, and not a preferred starting point for newbs.

Myriad of options out there for a suitable integrated amp that will easily best the HK include inter alia, my budget fave …the REGA BRIO integrated amp.

https://www.whathifi.com/rega/brio/review

To save money, buy pre-owned …. Many other quality brands pre-owned out there that include ROTEL, NAD, ARCAM, CREEK, CYRUS, ROKSAN , MUSIC FIDELITY, LINN, etc.

https://www.usaudiomart.com/classifieds/56-solid-state-integrated-amplifiers/?filter=FOR+SALE&province=&price_min=&price_max=&photo=N&show_msrp=N&show_other_marts=N

All components have their own bespoke sonic signature , so formulate a proper plan FIRST that also includes a preferred source, speakers, and cables. System synergy matters … full stop.

for the REGA BRIO, think preowned REGA R1/ RS1/ RX1 model bookshelf speakers (all preowned) for a dynamite system.

https://v2.stereotimes.com/post/the-rega-r1-loudspeaker#:~:text=The%20Rega%20R1%20becomes%20my,what%20more%20could%20you%20want%3F

For example. the NAD amps typically have a darker presentation with a rolled off top end. Not all speakers work well with it. (One that does is PSB speakers) The MUSIC FIDELITY can sound bright with certain brand speakers etc.

Roll up your sleeves , read some reviews, do some auditions , and engage your plan.

A lazy man’s default to Best Buy is a banishment to mostly low-fi, maybe lucky to a low-end mid-fi, and regrettably not hi-fi. Choose wisely.