I think I’d shoot a little higher than the P6 if I were you. Here’s a nice Bryston BP6 that’d certainly up your 2-channel game quite a bit (I used to own one) and is well under your $2k budget. It doesn’t have an HT Bypass, but you really don’t need one — just plug the front L/R preouts from the Marantz into the “Video” (or any other unused input) and balance the HT channels with the Marantz as usual with the Bryston’s volume at 12:00. When you want HT just switch the BP6 to the Video input and put the volume knob at 12:00 and you’re all set. This is being sold by a reputable seller who I think allows for returns, so if this doesn’t blow you away you can likely just send it back so very little risk. Anyhoo, here’s the link...
https://www.audiogon.com/listings/lisacbia-bryston-bp6-stereo-preamplifier-bp-6-black-19-32990-solid-state
https://www.audiogon.com/listings/lisacbia-bryston-bp6-stereo-preamplifier-bp-6-black-19-32990-solid-state