Here I have found a radical opinion about vintage amplifiers. Let me post a summary taken from "select45rpm". This is really extreme, but intriguing:
The Best Amplifiers & Receivers by era. ↑
The valve era appears to have the best in the Japanese made Sansui, Trio & Pioneer receivers, if they are still quite limited in spec. The USA McIntosh, Fisher & Sherwood appear to be of high quality but aren't in the UK usually to try. The UK brands like Leak, Lowther, Rogers, Quad as well as other small brands can be nice though they are less advanced than the Japan & USA product. Look for 10-20w for the best quality sound, an amplifier of 15w equals about 60w in transistor volume. But generally valve amps pre 1970 are nice but not as good as you hope they'd be...
By 1965-68 Transistors take over. There are very few receivers of 40w-45w & we seem to have found the best ones are the Japanese brands as noted below. Amplifiers of this era are only really the Sony TA-1120(A) to go over 40w as our List Of Amplifiers page shows. Some of the best amplifiers are receivers with the tuner, such as Sansui 3000A & National Panasonic SA-65. Others like early Trio-Kenwood & Pioneer we've found enjoyable. The only problem with these is the age, an amp pre 1969 is best used fully rebuilt & upgraded, to use them raw & aged will have them seenm to be not very good.
By 1969-72 Hifi is onto it's 2nd & 3rd generation in transistors. The Sound can be of high quality though the richer bassier valve sound is not around now. Still enough decent 40w+ amps & receivers though you will find cost cutting on some brands. The highest rated domestic Hifi beyond Sony pre & power combo is 65w for the Teac AG-7000 & Sony TA-1130, both are excellent amps which we've had. The Sony TA-3200F power amp is 100w, the Sony STR-6200F is 60w. Generally these are Capacitor Coupled designs if some Semi-Complimentary Direct Coupled ones are around. The quality is still good in this era, the era of wood cases with real wood veneer shows that cost cutting hasn't happened yet.
By the 1973-77 era Hifi gets into Discount Stores & the Cost Cutting Starts. This is pre the Receiver Wars era of 1978-79 and due to economics, there is a lot of budget hifi of not much quality starting around 1971 & by 1973 there are so many rubbish cheap amps sold it's not easy to see the better amps as few stock them. Some brands dip in quality with Sony & Pioneer generally making midprice quality with heavy cost cutting. Sony go low power & cheap by 1972, JVC-Nivico waste time with Quadraphonic & never recover the earlier quality. Pioneer suffer harsh cost-cutting as they sell well at big discounts, a foolish waste if a money maker. There are exceptions and Yamaha thrive in this era with many fine amps & receivers, they stay out of the Discount stores from their 1971 midprice range & the clasic 1973-78 range. By the 1976 era there are amps of 100w being introduced & from what we've noticed looking at circuits, there can be the odd top range gem amid the lower spec averageness. We've rated Pioneer as being heavily cost cut & the SX-850/950 sound rough & very limited in spec. But then we find the big SA-9000 series amps are very different, with the SA-9500 at 80w has a very good simple circuit, but cost-cutting is obvious, but the SA-9900 at 110w goes too much into overdesign & the later SA-9800 is ridiculously overdesigned. At this point by 1976 the top of the range item is overdesigned, the lower ones are cost cut with ICs but the one second to the top can be the best one. Also with Yamaha, the CA-800 is a nice design but the CA-1000 starts to get overdesigned resulting in the poor CA-1010 that is 100w but way too overdesigned. The CR-2020 can upgrade well but the amp as original is too warm & unfocussed. The CR-3020 looks way overdesigned too.
By 1978-80 the Receiver Wars are the game & much insane overdesign with the need for 300w is the deal. 300w is to drive very inefficient small speakers that need high current, totally against our ideas of Hifi. By this era many look the same, garish design based on the Pioneer SX-850 & even UK Leak copied this style & got theirs built in Japan. By this era Yamaha dipped in quality with the CR-2040 range having many ICs in the audio, Pioneer's SX-980 goes similar. The Receiver Wars were over as the 1980 HFYB proves as even Marantz go back to lower powered midprice gear after their excessive 300w & 340w receivers. By 1981 the sort of Hifi you could still find in 1991 was the deal, if silver was now black & progress in Hifi appears to have got even more cost cut with higher power as the Amazon & Richer Sounds type amps show. After 1980 there are an amount of 'exotic' type amps, many stuffed with ICs that claim to be better Hifi than they are, delivering a cold boring sound. The 1965-77 era is the best in Vintage Hifi, even buying a modest 25w will sound so much better than the majority of post 1980 gear.