I have practiced 3 musical instruments for 30 years....owned tons of gear.
1. The value is incredible. This is a 190 watts power amp that fits in your hand. This is a similar design to the Seymour Duncan Powerstage, at a fraction of the price! I have other Harley Benton gear, one isolated power supply and one attenuator. All this Harley Benton gear exceeded my expectations, this is THE best lower cost, professional gear you can buy.
2. This is PERFECT for tube preamps or hardware amp sims. As oppose as other designs such as tube power amp, which colours the sound, which now I do NOT like anymore, the GPA is a class D power amp. Basically, this is a HI-FI power amp for your preamps. You do NOT want class A or class AB power amp for your Pod, if you want the power amp to reproduce EXACTLY the tone you hear in your Pod when you use it with headphones.
In the past I swore by tube power amps....now I do not! I STILL use ONLY tube preamps (not Pod or amp sims) but I am definitely a class D power amp convert. I do NOT want the power amp to colour my preamp. This is an old debate with tube amp guitarists. What happens with a tube power amp is that as you increase the volume, the bass and treble will roll off, and the mids will increase. But today I do NOT want this anymore. Today I want BASS, a modern metal tone. I don't want the bass to roll off! I want the power amp to reproduce EXACTLY the same tone, at ANY volume. THIS is what Class D power amps such as the Harley Benton will do. Also, a tube power amp will distort sooner. Why? Because tube power amps are more expensive, therefore it means less power for the money. But class D doesn't have this problem: Class D watts are far cheaper, which means you get more power, which means more HEADROOM, and so the power amp stays clean. Plus, it is a FAR MORE efficient form of amplification. Almost 200 watts in your hand! With class A or class AB whether tube OR transistor, this would result in much bigger and heavier gear (you need bigger components than class D), etc etc, and this adds to the WEIGHT and the COST. This is why class D power amps cost less. But 75 euros for 190 watts rms? Incredible.
3. The GPA has a powerful ACTIVE eq. So you can add as much bass, mids or treble as you want, AND you can also CUT these frequencies, which you cannot do with PASSIVE tone controls, found on MANY power amps that are a LOT more expensive than the GPA. So basically the GPA is the PERFECT power amp. Some people lamented the fact that the GPA doesn't have resonance control. But ALL power amps have resonance, in the GPA it is just fixed. BUT there's ACTIVE bass control, which is BETTER than resonance control, which is PASSIVE and thus weaker. The bass in the GPA can take your head off, but it doesn't get boomy. Also the treble doesn't get harsh. This works PERFECTLY with my tube preamps.
The GPA will NOT sound inferior to the Seymour Duncan. And it's made solidly, of metal. This is not some cheap Chinese rubbish. The GPA is perfect for pros too....you don't always have to buy famous brands. Frankly you'd have to be dumb to prefer the Seymour Duncan Powerstage to the Harley Benton GPA 100! They both do the same things!
4. the GPA will tell you if the preamp output is too loud and distorting the input of the GPA. It also has a protection circuit in case of impedance mismatches at very high volume. But if you use correct impedances, it will probably last a long time, like any quality gear.
5. With my Blackstar HT Metal tube preamp my setup is pretty much NOISELESS, even with Ultra gain! I usually use noise gates, but I don't need it now. Just incredible! And the tone is amazing!
CONS: Absolutely NONE. A tube power amp does NOT sound better to me, only a tube PREAMP sounds better than non-tube preamps. I have made extensive tests in the past, for example I tested my tube preamp into a tube power amp and a solid state power amp, and the tube power amp did NOT sound better. But both tube power amp and solid state power amps WILL colour the tone and compress it at HIGHER volume. Because they are all class A or AB. But Class D, as used by the GPA , is MUCH more transparent than class A or AB. Like I said, basically a HI-FI power amp for guitarists. In my case, this is EXACTLY what I want. But the other advantages, such as ridiculous price and size and weight, are huge bonuses of course. Class D power amps are the BEST! (If you have a great preamp!).
So be smart and BUY the GPA!
(EDIT: after further investigation, the claimed power rating (100 watts or 190 watts with the other impedance) is not RMS, which means the effective power will be less. However I am pretty much sure that the GPA 100 can put out at least 50 real watts, which is still a lot and surely enough to cover most gigs! so it's STILL a superb bargain. I now used it for several weeks and I absolutely love it! The active EQ is so powerful, even with a thin sounding preamp you can get a ballsy tone as you want, even turning the EQ a little bit really sculpts the tone. Or you can leave it neutral and get a HI-FI amplification for your preamp, at any volume.)