I'm looking to replace my aging HK AV255 with a newer receiver, preferably one that’s networkable.
The models I have for consideration are….
Sony STR-dn840, the Denon AVR-X1000, the Onkyo TX-NR525 and the Marantz NR1504.
I don’t really need 7.1 or 7.2 and really like the slimline look of the Marantz, but am just a bit worried about the small size. Has anyone any personal experience of any of these models?
Thanks in advance.

The Marantz has been around a while. It owes its smaller size to the fact that it uses Class D power amp technology. This does away with the large and expensive transformer and power reserve capacitors of traditional analogue amp designs and replaces them with a high-frequency switch mode power supply. . It is directly power from the mains in to music wattage output. Class D amps have been around a long time. They're used in car audio, in AV subwoofers and also in all-in-one home cinema kits. But it's only in the last few years that developments in Class D technology were considered sufficient that it could be used in more upmarket AV receivers.

Marantz were one of the first to make a virtue out of the smaller form factor of Class D amps in a higher-end AV receiver. Now though, you'll find Class D in many AV receivers up to £600. IMO this isn't a positive development. Class D has sonic trade-offs. However, there are no reliability horror stories with the Marantz, and since everything else in the same class also uses Class D amps then it's Class D or nothing.