The amount of power you need to drive speakers is determined by the following:
1) The efficiency of your speakers expressed as decibels/1watt at 1 meter measuring distance from the front of the speaker. Most bookshelf speakers are going to fall in a range of 86~90db/1watt. Two speakers that are 3db apart in efficiency means that the less efficient one will require 2x the amount of power as the higher efficient speaker to play at the same acoustic level.
2) The second consideration is how loud you like to listen to your music. In a home, most people would find an average acoustic level of 85db to be moderately loud. Turning up the volume just 3db (barely noticable to most people) will require 2x the amplifier power.
Peak levels, depending on the type of music, can run 10-15db higher than the average levels. This is true of classical orchestral music. Most rock music is highly compressed so the peaks on this type of music are much lower, probably no more than 6~8 db. A 10db peak will require, for a brief fraction of a second, 10X the average power level. So if your music and listening preference require 1 watt average, your amplifier would have to be able to deliver 10 watts on the musical peaks. You can think of the power ratio of average to peak power demand in the following way: 1 watt:10 watts, 2 watts:20 watts, 3:30, 4:40, 5:50...10:100. You can quickly see how listening at even moderately loud levels where the average power demand is 4 watts will require at least 40 watts to be able to reproduce the musical peaks of wide-dynamic range recordings without clipping.
Fortunately, brief clipping of the output signal is usually not audible when listening to music. Also, many amplifiers can deliver, for short fractions of a second, output power that is 2X or more the continuous RMS rated power. NAD made much of this with their receivers and amplifiers, where typically 3-4db headroom was designed into their products, so their 20 watt receiver could, during musical peak demands, deliver 50 watts of power without clipping.
3) The last factor would be the size of your listening room and its furnishings. In the typical home, any bedroom or den would qualify as a small listening space, any living room or family room as an average size room. In really large rooms, additional power (perhaps 2X) would be needed to compensate for the additional space of the room and the ability of the carpets, drapes, and furniture to soak up sound waves.
In terms of a vintage receiver line such as the Marantz 22xx series, these were available with power ratings from 10 - 70 watts RMS power per channel (later models offered still higher power). The most popular models sold were in the range of 15 - 40 watts. Although the top of the 22xx line Marantz model 2270 was rated at 70 watts RMS power at 8 ohms, and could actually deliver 100 watts at 8 ohms, it had difficulty driving 4 ohm speaker loads* and is not considered the most musical sounding of the Marantz receiver line. The best values today in used Marantz receivers are models such as the 2215/2215B, 2220/2220B, 2230/2230B and 2235/2235B. These show up quite often on eBay and rarely sell for more than $100.
* 4 ohm speakers require an amplifier to deliver 2x the amount of current to the speakers as do 8 ohm speakers, for the same amount of power. Some amplifiers are more capable in this area than others. The Marantz 2270 was challenged in this area, the later 2275 was better able to drive low impedance speaker loads. Many late 70s and early 80s solid state designs can actually deliver 2x their rated power at 4 ohms vs. 8 ohms. They can do so continuously not just under peak power demands. Most tube amplifiers are more suited to driving (higher impedance) 8 or 16 ohm speaker loads than (lower impedance) 4 ohm loads. Tube amplifers are primarily voltage sources, with current delivery a secondary attribute. Assuming the tube amplifier has output transformer taps for 4/8/16 ohms, any speaker can be used, but the 8 or 16 ohm speakers will make less of a current demand on the amplifier and it will run cooler, hence the tubes will last longer.