Preamp and impedance question

@erik_squires Currently, he will use Tango XE-20s transformer.  I hope it is a good quality transformer.  But if I want to have a 4 ohm speaker output, I need to add $500 to upgrade to Tango FC-30-3.5s.  So, for the additional $509, I get a 4ohm output and likely better sound, but don’t know how significant it is.  The ultimate question is, should I do this upgrade .... I think I don’t need 4 ohm.  My current speaker has a power subwoofer.  So, it seems the dip in impedance at low frequency is not a concern for now.  but I may change my speaker, or I may sell the amp.  To ask the question a different way, when you buy an amp, is it important to you for the amp to output 4 ohm?

@georgehifi 
thats good news that I can just add the volume pots. =)

my current speaker is a Spatial Audio X5.  It has a power subwoofer.  When I say my speaker dips to 5 ohm at 30Hz, it is for my old Zu Omen Def.  And that is based on my memory, so, number may not be exact.  I just use that as an example, easier to give some numbers.

and I remember when I use my Zu, I feel the 4 ohm tap gives more bass.  But it can also be a psychological impression, nothing scientifically measured.

Spatial Audio X5

Na, nothing measured on these, but just by the looks of of them, I'd say a 4ohm tranie tap will be fine for these, btw the real Lux has 16-8-4ohm taps on it's transformer.
Cheers George

@georgehifi yes, can’t find the curve for the X5.  Actually, the current price without upgrade only have 8ohm tap.  In your opinion, is 8 ohm ok, or is it worthwhile to add $500 to upgrade the transformer, which will then have a 4 ohm tap and likely will improve the sound?  

While reviewers tend to prefer 4 Ohm taps, the reason for these taps is to ensure equal power output. My mind is hazy, but I think the main reason for this is that without the tap your amp’s power output drops at lower impedances.  @atmasphere is the real tube/transformer expert here.

Best,
E