I owned both the NAD C370 integrated amp and NAD C420 Tuner in a prior "B" system.
(1) There is nothing "obsolete" about either of these pieces performance-wise ...in fact its argued in many reviews that the C370 intergrated amp was one of NAD’s best offerings besting even the the 372 and later models.
(2) "NAD kit is "obsolete" and go move to a SONOS system...."?? Other than wireless capabilities that can be matched with adding on an external wireless DAC etc as an add-on to the C370 for streaming etc , then this assertion sounds like just a hardsell misguided self-serving sales pitch...full stop.
(3) Your existing source with a SONY multi-disc player can certainly be upgraded SIGNIFICANTLY without blowing the wad.Yes.... this one piece is obsolete. I would get say, a matched single-spin NAD spinner and/or or NAD external DAC to complete the current technology add-in, the all-in-one matched appearances issue, and the synergy issue.
(4) All AV receivers are "obsolete" within months of release with ever-changing CODECS etc. AV Receivers are also all just made-to-their price-point quality (usually very dodgy crap-build to the cheapest price-point possible Chi-Fi kit) that simply don’t measure up to quality built integrated and power amps because they lack the high-end power supplies, caps etc. ... With the possible exception of the very top-end ARCAM FMJ AV receiver. Yes, the Marantz AVR is also obsolete as an old 6.1 tech gear circa 13 years ago.... But that is "AV" and not relevant to any 2-channel NAD kit herein.
(5) Unless you are just looking to sell out completely, choosing speakers with the desired requisite synergy with your NAD is the issue for 2-channel. I know the NAD strengths and warts very well having directly owned them. PSBs immediately come to mind commensurate with your NAD gear price-point.
(6) if you are looking to sell:
the NAD kit:...
C370: $300 +/-
C420: $100 +/-
Me...I’d keep the NAD kit, and go get a new source (CDP and DAC) and also get new speakers to do them justice cuz at those modest price-points, they are good performers that wont be easy to match much less beat in performance by any of the many budget level contenders and pretenders out there.
"..... It’s Your Money
The NAD C370 ain’t long on style, but I found it alive with substance—a real blue-collar champion offering an honest day’s work for an honest day’s pay. Its smooth, authoritative, solid-state sound was delivered with conviction: warm, dry, and solid, never edgy or fatiguing.
If you’re looking for more personality and refinement, you’ll have to dig deeper in your pockets. However, as the flagship offering of NAD’s high-performance/high-value Classic Series, the C370 is an exceptionally solid, versatile, musical performer...."
Read more at
http://www.stereophile.com/content/nad-c370-integrated-amplifier-page-3#oiK5QAXrGuWhaWB8.99 The SONY multi-spinner .... negligible to minimal value in today’s marketplace ...mostly for spare parts value
The MARANTZ SR 5300 AVR...yes, this was also an old 2003-era 6.1 budget receiver for $600 in its heydey. Its probably going to fetch minimal to sub-$100 in today’s private marketplace. Dealers won’t touch it generally.
Good luck