Joined: Nov 16, 2001
Posts: 2053
From: Mpls, MN

In most cases, you are probably not going to want to own a Class A amplifier anymore, and especially a Tube-typed. Tubes are much higher in distortion, and squelch the top & bottom frequencies from your music. Class As are very, very clean, but to the human ear, an AB or Class T is clean enough. 
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Efficiencies? 

All amps are very efficient when driven to 100% THD (fully clipped). However, what is important is how efficient they are at normal RMS ratings. Class Ds & Ts are generally 70% efficient, ABs 50%, and A & Tube 33%. Basically, you'd probably want Class D or T on subwoofers--even though Class Ts are designed for full range. 
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Class D vs Class T? 

Which ever one most affordable and reliable. 

Can you hear the difference between Class A, A/B & Class D on subwoofers? No 
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How Much to Spend on amplifiers? 

Rule of Thumb: A Buck A Watt 

If an amplifier's retail price divided by the regular RMS output is at least $1/watt, it is a worthwhile product. EX: 

Rockford 800.2 200Wx2 @ 4 ohms $700 retail 

$700/400W = $1.75/W 

Memphis ST-1000D 300Wx1 @ 4 ohms ~$700 ret 

$700/300W = $2.33/W 

Both are worthwhile products! Keep in mind, there are 3 types of manufacturers: 

1. Ones whose products exceed their ratings 
2. Ones that meet their ratings 
3. Ones that do not meet their ratings. 

The purpose of the $1/W theory is to tell you which ones meet or exceed their ratings. The logic is that if they don't meet their ratings, no one could charge (or would be willing to pay) more than $1/W. 
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Why not go over $3/W? 

Because a Mercedes will get you to the grocery store the same way a Toyota will. Above $3/W and you will NOT hear an audible difference compared to justify the cost. This is not to say that you may not be able to MEASURE the difference, but do you really care. 
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Amplifier Specifications--Important? 

Distortion gives us a relative measure between the desired output, and the undesireable output. 1% THD+Noise means that 99% of the sound ouput is desired. How much is acceptable? To the Human ear, probably 3%. 

RMS/PEAK power tells us what the manufacturer rates their products output. Ratings of 12-13 volts input power are meaningful since those are voltages that a factory alternator is capable of operating. Anything more is probably not valuable. 

Frequency response? The bottom & top ends of the amplifier's capability. KEEP IN MIND that class D amplifiers are NOT intended to be used on anything other than subwoofers!!!!! Class T (basically still a class D, but with the TRIPATH ® chip) are designed to work full range. More on them later. 

Slew rate, Damping Factor, other specifications are really not that important if you consider the $1/W theorem. Damping Factor over 25 is good enough and a higher number is not likely audible. 
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Crossovers? 

An 18dB/Oct is great, but 12dB/Oct is good enough for most applications. Variable crossovers that go from 30Hz to 100Hz for subwoofers is adequate, and 50hz to 400Hz is fine for High Pass. 

Are External crossovers, passive crossovers (chip type) better than those built in amplifiers? No 

It's possible that an external crossover that uses 'chips' rather than variable potentiometers is cleaner, but it is not possible to hear it.