HD-5000, you did not say how the amp will be used. In some applications, 20 watts/channel will be more than enough power, in other situations, 200 watts/channel will not be enough to drive loudspeakers to required acoustic output levels.
I suggest you decide on your speakers first, then determine how much power they will need in your listening room. This is partially determined by the efficiency of the loudspeakers, by the size of the room, and by the type of room furnishings. Also, the type of music you prefer along with your listening habits will determine how much power is needed.
In general, the only things that an integrated amplifier gives up to a separate amplifier and preamplifier is the maximum power available, placement of the power amplifier in relation to the speakers, and possibly some switching flexibility when compared to a dedicated preamplifier.
Although there are examples of integrated amplifiers capable of providing more than 150 watts/channel into 8 ohm loads, typical integrated amplifier is 100 watts/channel or less. Many integrated amplifiers are in the range of 30-70 watts/channel. This may be plenty of power for your particular application, or you may need more power. Remember, to gain any audible benefit from a higher power amplifier, you must double the power. So the minimum upgrade in power from a 30 watt/channel amp would need to be at least 60 watts/channel. Any less just won't make an audible difference. A 300/watt channel amplifier can provide 10x the power of the 30 watt unit, but it's maximum level will sound only twice as loud to the listener as the lower powered one, not 10x louder.
There were vintage integrated amplifiers that sounded great, and some that sounded so so, and some that were pretty much crap. Same is true today. The vintage 70s amplifier and integrated amplifier manufacturers that were the most highly regarded in their day included:
Harman Kardon
Luxman
McIntosh
Marantz
Pioneer
Rotel
Sansui
Technics
Yamaha
Technics and Pioneer had some good products, but also much of their stuff was mediocre.
McIntosh was known for the excellent separates, much more than for their integrated amplifiers and receivers.
Yamaha had a full line of integrated amplifiers, including several models that could be switched to run in full Class A (non switching mode), although with much lower power.
Rotel has been designing audio components in the UK and manufacturing product in Japan for more than 20 years. It is not as well known as the other brands, but Rotel products are consistently excellent quality audio gear.
Marantz, great sounding solid state gear without the harsh or brittle edge that sometimes comes with solid state audio. For the money, you can't do better than a well maintained piece of vintage Marantz including their integrated amplifiers and their receivers as well as the separate amplifiers, preamplifiers, and tuners. If after trying a Marantz unit in your system, you find that really don't like it, you won't have any trouble finding a buyer for this gear.
Sansui offered a full line of integrated amplifiers, at a price that was considerably less than Marantz at the time. Worth considering is their mid-70s line of discrete solid state component equipment such as the AU-5900/7900/9900 series.