Current vs. Watts??? Need clarification.

I've got a Rotel RB 1070 PA (about 125 watts) driving Spendor S8 speakers and I can't escape the feeling that this amp doesn't deliver enough power to drive these speakers. I'm getting clipping at pretty moderate volumes - say 1/3 on the volume knob. I've heard that this may have to do with the level of curreent actually being delivered to the speakers, as opposed to the amp's stated wattage output. I have read that the S8s like a lot of power but I would have thought 125 watts sufficient to drive a 2-way Is my amp just too small? Anyone have any thoughts on this?
That seems unlikely... They are 89 db efficient with 8 ohm impeadance, and unless they drop below 2 ohms or something when driven you should not have much issue.. Sounds like either the speakers are blown and show distress only at a certain volume, or the amp is having issue, or your preamp signal is bad coming in and maybe shows it at a certain gain.. Bad volume pot or something? I don't know much about the amp or speakers you have but from the specs it would seem they work fine to realitivley loud output, and would probably shut down the amp into a thermal protection or something well before it would clip to easy.
I don't have much experience with any recent vintage Rotel equipment, but there are a couple of potential issues here.

1. "1/3 on the volume knob" is not a reliable indicator of anything. If the source is pumping a fairly high line level voltage into the amp, a 1/3 setting could be asking the amp to deliver near it's maximum power. A better question would be what type of dB volume in the room are you hearing when you experience this clipping? If you are hearing clipping at 85 or 90 dB, then there is a problem somewhere. Either some component is defective or misconfigured. If you are listening in excess of 100 dB, you may well have nothing more than a power issue. Radio Shack sells an inexpensive sound meter that will help you determine this.

2. That said, some amps can have trouble delivering their rated power at lower impedances. The Spendor S8 has a nominal impedance of 8 ohms, but a minimum of 5 ohms at some frequencies. I wouldn't think that should be a problem for a reasonably well designed amp, but some do have power and stability problems as the speaker impedance drops.

The formula for wattage is voltage times current. However, on a transistor amp, when the impedance drops from 8 ohms to 4 ohms, the speaker is asking the amp to deliver the same voltage, but twice the current. Some amps aren't designed to operate well at lower impedances.

Anyway, I'd think the first issue is to determine just how loud you are really playing the stereo. Once that benchmark is set, you'll have a better idea if your amp is misbehaving or you just need more power or a different amp.
The basic question goes to the understanding of Ohm's Law, V=IR. Where V is voltage, I is current and R is resistance. It should be clear from the formula that, as the impedance of a loudspeaker diminishes, current must increase if voltage is to remain the same. Since the speakers probably do not have a very punishing impedance load, what type and how long are your speaker cables?

Ultimately, your question probably has a more pragmatic answer. Your speakers are probably not very sensitive, which requires more wattage to produce a given sound pressure level. Even though the spec says 89db, which is sensitive, different measurement tecniques yield differet figures. As an example of Spendors optimism, they rate the S5e as being 87db efficient, yet Stereophiles testing shows an extremely low sensitivity of 82.5db. see here,
That 4.5 db does not seem much but it would require over three times the output power to achieve the same volume level. The answer to your question is that more amp power will certainly help if low sensitivity is the issue. But many of these classical British monitors are rather polite and have a lower ultimate loudness level than others, so they just might not play that loudly.
Are you driving one set of speakers or are you driving two sets simultaneously?
Just to add to the mix that pre-amp overload may also play a part. Sometimes some form of attenuation is needed between source and pre. Especially if you are only getting limited volume adjustment.
Gentlemen, That is a great set of answers. Thank you.

To clarify a few things.

1. Beavis - I'm just driving the one pair of speakers. There is only one set of posts on this amp.

2. Viridian - It is interesting that you mention the S5e, as I actually had that speaker in the house for a month or so before trading up to the S8e (not enough bottom on the 5s.)and I did experience the same problem on at least one occasion.

3. Mlsstl - I have to confess that 1/3 is up pretty high on this system. I sometimes - foolishly - try to make up for either ambient noise or lack of detail within the system itself by turning up the volume. Although, as I've lived with the system for a while I learned it's limits better.

Thanks again. G
W = E X I X P.F.

P.F. (Power Factor in %) refers to the phase shift between current and voltage at various frequencies. Some speakers are more prone than others.

So, the more phase shift you have the more current required to achieve the rated watts.

A sufficient power supply will help sustain the voltage during larger phase shifts, and some amps are able to deliver more current.

I guess there are a lot of ways to skin a cat and some amps do it better than others. :-)....?

In theory, an amp rated at 128 wpc, will deliver a sound level of 110 dB at 1 meter when paired with 89 dB speakers. However, it will only deliver about 104 dB at 3 meters...which is where most of us listen to our music.

Also, at about 60% of the amps rated power, distortion will start to creep in. Consequently, we must reduce the theoretical output of the amp by another 3 dB if we want clean , undistorted sound.

Conclusion = Not accounting for the many other factors which may further reduce the sound level (listening room acoustics, age/condition of components, headroom to cover peak dynamics, etc., etc.), we will be lucky if our 128 watt amp delivers 101 dB of "clean" power at 3 meters.

I always heard that a 50 watt tube amp puts out the same current as a 100 watt solid state amp because of the higher voltage. Is there any truth to that?
Tube amps do not deliver a higher voltage to the speakers. The higher voltage from the tubes is run through an output transformer to send the correct (lower) voltage to the speakers.

Many people ascribe the tendency of tube amps to sound louder than solid state amps of the same wattage to a difference in clipping properties when a tube amp is overdriven. Tubes tend to have a softer clipping characteristic so don't sound near as harsh and distorted as when a transistor amp clips.